I've heard that sports and wildlife photographers need cameras that can take lots of photos in a second in the burst mode.
But why? Yes, simple question — but why?
Strictly speaking, one does not need high FPS burst modes for sports or wildlife, but rather they are useful tools that open up more options.
I've shot sports in the last few years with a Canon 7D, typically using 5-8 frame bursts (at I think 8fps) at a time, and I've also used a medium format manual focus camera.
Both methods have produced great images, but with a burst of photos I walk away with a number of images I can choose from that best represent the moment. (ie, picking between two nearly identical photos, one where a player is halfway through blinking, or one where they're not...)
When I have only a single frame to pick from, that's it. That's the one frame, and either it worked well, or it gets tossed to the bin, because there were no other options to pick from.
Burst mode, with a large frame buffer, can also help settle some minor motion blur - It isn't unusual for the first image or two of a burst to have a hint more blur than the rest from pressing the shutter button (and parts in the camera starting to move in an SLR) but the following images are balanced out.
For me I find this especially useful during panning shots - Begin following the target, anticipate the action you want to capture that is about to happen, begin a burst, and follow through across your goal image.
When shooting sports and wildlife you don't really have the luxury to stop and say "Lets try that one again" if you don't feel you nailed the image you were after.
Options then to be good.
You can always take a single frame on a modern camera that can do 10+ fps. It is 'kind of hard' to take 10fps if you have a camera that only does less than 1.
(As a side note, also remember to consider buffer size and clear times. A camera that can take 10fps won't do you much good if you frequently find yourself tapping the shutter button and hearing it beep at you that it is still clearing its buffer and can't take a photo right then.)
In addition to all the correct answers about how fast action occurs, I'd like to point out two fundamental biological reasons for why you need burst:
If you get into slow motion, the effect is even more extreme. If you've seen the shark episode of Planet Earth, you've seen some of the extraordinary footage they capture. They don't actually have a shutter button. They turn the camera on and it starts rolling with a 2 second long FIFO buffer, meaning the camera always remembers what happens in the last two seconds. After an event happens, they would press a trigger to latch the buffer, remembering those last 2 seconds until they got the chance to download it on the boat. So I would call that the ultimate spray and pray -- they continuously take pictures until something happens, then tell the camera to bother keeping the last few!
For a specific case where a high FPS is useful, consider cricket photography (the game, not the insect).
When a batter takes a swing at the ball, there are potentially a number of outcomes worth taking a photo of:
The optimal time to take the photo is slightly different depending on the above outcomes so by taking a few shots in rapid succession covers both.
I acknowledge the comments about the "spray and pray" approach, so choosing when to do this comes down to judgement.
I've been shooting at Motorsport for a while now and I can give you my vision of why this is like this I believe in any sport.
Usually sports action occurs so fast that any image can change in a fraction of a second. Take again the image of the Motorsport races. Sometimes a car hits another one and they go off track. One image of that is okay. But you always want to have better and more images, so you can get all the details of what happened there, who caused the accident, and so on. It is not professional to get an image of the car parked once damaged. The thing you need to have is to get the crash in an image or more than one, and this is why a high burst rate helps. I got some crash/accident images because of that, and I was able to track the whole (or mostly) action to later capture the best images showing what happened there. (let's also not forget AF in cameras also tend to somehow miss focus sometimes, so if you take 8 images and 3 are blurried you are okay with 5 more, but if it bursts at 4fps and you got 3 blurried images is not that good at all).
Let's put another example: A race and a chicane where not every car jumps there on every lap, but randomly sometimes, some cars do. You do wanna have that image when one car will fly there. You don't know when would that happen, but you know this can happen and this is why you wanna have a nice burst rate, to capture everything that happens there. Maybe you will not get that image. But if it happens you wanna capture that moment.
You can perfectly fine shoot motorsport with a slow burst rate, and you'll get also great images, but most likely you'll loose that image.
I believe in the rest of sports is the same. If you are picturing while a 100m spring race you do wanna get the winner's face perfectly defined when he/she does that gesture with his/her face. Or if two runners fall hitting each other, you do want to capture exactly that moment nor the previous one, nor the one just after. And having a high burst rate help you getting the image.
I'd also say is not only fast burst rate, is also accuracy and fast AF. Quite franckly the most important is the AF. No one needs a wonderful collection of 8 images all out of focus. So I'd say is more a combination of great AF tracking and high burst rate.
They don’t know WHEN the moment they want will happen. WHEN will the birds wings be FULLY extended? WHEN will the ball be ON the racket/bat?
Answer sometime in the next second. If you have 4fps that 4 tries 11 fps 20 fps are more shots. You have more images to find which is closest to that KEY moment. High jumper toe JUST leaving the ground
Its not just the shutter speed to freeze action, (iso and Fast lens (wide aperture)) but Also many images to try get one at key moment
Question: Why exactly do action photographers "need" high fps burst cameras?
Answer: In order to shoot as many photos as possible in a given amount of time and/or to reduce the time between shutter actuation's.
Sometimes in fast action sports or high speed animals the moment you want to capture happens at the same moment the camera is resetting the shutter and you miss it.
This answer was created by a high APS (answer per second) burst Alaska Man.
Most of the other answers are perfectly valid. Another aspect to be considered is that -each- of the frames can be sold individually and exclusively to different buyers. You are selling the image, not the event.