I just wonder that point and shoot cameras are great, but why professional photographers use DSLR.
To start, saying that "professional photographers use DSLR[s]" is a bit of an over-generalization. It is not too far from the truth, though, to say that professionals rarely use fixed-lens compact cameras (which I'll use in preference to "point and shoot" here; I'll explain that later), but even that depends on what you mean by "rarely". To a degree, it also depends on what you mean by "professional photographer" — it's very easy to forget that there are a lot of professional photographers who don't fit into the "wedding and event" or "studio and location generalist" categories.
If we can consider "DSLR" and "large-sensor mirrorless interchangeable lens camera with an eye-level electronic viewfinder" to be more or less the same class of camera for the purposes of this discussion¹, then there is an awful lot of versatility and bang for the buck to be had in this class. For a given resolution, the large (APS-C or larger, compared to the 1" or smaller sensors in most fixed-lens cameras) sensor means that you need less light to acheive the same signal-to-noise ratio. Since there is a definite "too noisy" level for results that someone is paying for (it varies with the job type, but there's always a limit), that means that the larger-sensor camera will let you shoot in more different lighting situations. The larger sensor also means that you get greater control over how much of the picture is in sharp focus (and how far out of focus the out-of-focus areas can become) since you will be using longer lenses and focusing closer (relative to the focal length) to get the "same" picture.
Starting at around $1000 (in the US market, with a few notably cheaper examples), you can get an APS-C-sized body that offers direct manual control over all of the "shooting time" settings, usually without having to take your eye away from the viewfinder (once you're familiar with the camera). There is no question that $1000 isn't quite pocket change for most people, but for a tool that makes the difference between getting the shot and watching the moment pass out of the corner of your eye while you fiddle around in the menu screen on the back of the camera, it means you can still be a professional next week if you're an event photographer. You can, of course, spend more for things like more responsive autofocus, larger sensors, faster frame rates, less shutter lag, ruggedness, reliability and improved handling, but you really can't afford to spend less than the minimum it takes to ensure that you can get the shot.
Interchangeaable lenses mean that you can use the right lens for the job, and that you can both upgrade the lens and keep the lens when you upgrade (or replace) the camera. All lens designs are a compromise (even when the advertisements use words like "uncompromising" and the prices leave you gasping for air), and you get to choose what those compromises are on an interchangeable lens camera. It is possible to make a lens for a large-ish sensor that is, at the same time, a good recilinear wide angle at one end of the zoom range, a relatively long, sharp and fast telephoto at the other, a great performer everywhere in between, perfectly parfocal, and a decent semi-macro to boot. Both Canon and Angenieux have done it. Both want more than $40,000 for theirs. And if you buy either one, you will want someone to carry it around for you (both weigh around 8.5 kg). Price, size and weight are the compromises in those designs. You may wish to make others instead, like using shorter zoom ranges to keep the lens speed while reducing the overall size and weight of the lens without introducing too much distortion or uncorrected aberration. Or using a fixed focal length ("prime lens") to get wider apertures and/or shorter minimum focus distances.
An eye-level viewfinder means that you can hold the camera much more stably so that you can use slower shutter speeds when hand-holding. Image stabilization only adds to what you can do without IS; it can't make up the extra you lose by holding the camera at arm's length. (It would be worth noting here that rear screens are a young person's toy. They're at just about the wrongest distance possible for anyone who needs reading glasses or bifocals unless your hobby is shooting other people's feet or you shoot everything at waist level.) On a tripod, it won't make a difference (and may be better in some respects), but there are huge chunks of photography where a tripod is more hindrance than help.
So... for a reasonable price (for some value of "reasonable"), you get good low-light performance, the ability to use the right lens for the job (and your budget), easy access to the controls you need, responsive AF, little lag between the time you press the shutter button and the time the picture is taken, and stable hand-held shooting. I've probably missed a few things, but those are the highlights.
I did mention, though, that pros don't necessarily use DSLRs. A photojournalist or travel photographer who wants to stay discreet may very well use a point-and-shoot, and there are a few "stealth shooters" in the wedding/event trade who will do this as well (with more advanced compacts, while the more obvious and formal DSLR photography is being done by second shooters). At the high end of the architectural and product photography markets, pros will often use a view camera with a digital back to allow them sufficient control over perspective distortion and depth of field. A backpacking landscaper may very well find that something like the Sigma DPx Merrill or Quattro, along with the smaller tripods they would need, make the best compromise between image quality and the weight and bulk of the system. There are a lot of reasons why either more or less camera than the typical DSLR might be the best choice; these are just a few examples off of the top of my head. But do note that each of these is a sort of special-purpose use of a special-purpose camera; none of them is quite as versatile as a DSLR/MILC. If you're a generalist photographer, or someone who hasn't found a market niche yet, that would mean having the special-purpose camera in addition to something that is more suited to the jack-of-all-trades, and that's extra money that might not have been spent as wisely as it could have been.
¹ Until recently, I would not have put the two together, but EVFs have become actually useful lately, lens availability is better than it used to be, and on-chip phase detection AF mean that the down side of MILCs is pretty small these days, and it's really as much a matter of personal preference as anything else now.
point and shoot cameras are, by definition, rather bad cameras because you have very limited choice for the camera settings; the camera will make the choices for you, or these are fixed. In addition, the optics and the sensor are typically of poor quality. DSLR cameras allow you to be in charge of the camera when taking a shot, instead of the other way around.
The main advantages of a DSLR:
- You have a lot more control over the settings of the camera.
- The hardware/build of the camera is of higher quality (after a certain price treshold).
- You can change lens, add filters and accessories easily, it lets you adjust to the type of scene and picture you want for an optimal result.
- Better build/quality = better sensor and optics, thus meaning better result in the end.
(among tons of differences between the 2).
DSLRs are faster (mostly focus and write to card speed).
DSLRs have larger sensors (than p'n's) that will offer better performance in low light (less noise); have better depth of field control).
DSLRs usually have faster shutter speed to be able to take pictures of moving subject (sport events) and the lag between the moment you press the trigger and the time the picture is taken will be smaller.
They offer wide range of lenses; from zooms, to prime lenses from macro to wide angles to give the photographer the best tool needed for the job.
In general, DSLRs are faster in operation than point'n'shoot cameras.
DSLRs will give better and more control to the photographer when the conditions are not ideal (light, speed of subject, white balance control, frame rate, ... )
IMO, PnS cameras (including phone cameras) have their uses, and when used within their limitation will give result comparable to DSLRs (in good lighting, ... )
(addendum) Professional will use (high end) DSLRs because companies like Canon and Nikon offer professional support (quick replacement/repairs, loaners, ... )