Is an artificial external source of light required even if you have a camera with a high range of effectively noiseless ISO?
Are there some special cases which "need" external lights, at all costs?
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Lighting is about a lot more than simply making a subject visible to the camera, it can change a shot dramatically so always relying on naturally occurring light severely limits creative outcome. You have to bear in mind that high ISO shots rely on the available light and will simply make the scene uniformly brighter the higher you go. The available light may be perfectly adequate to get a correct exposure at high ISO but may be so even for instance that the resulting image is flat and boring. In the other extreme if available light is very low boosting ISO will just give a uniformly bright image with no tonal range. Introducing external lights will add contrast and depth to the shot if done carefully.
Well, as long as you need to change the way light falls upon your subject, you are going to need external lights. If you want to add or remove shadows that are not possible with the avilable light and a reflector will not give enough light, a light source you can reposition will save your day.
Well, results are completely different. So, depending on what you want, you will need one, the other or even both.
High ISO is extremely useful because it brightens everything regardless of distance. So, if you shoot people on location, both background and foreground will brighten compared to a lower ISO. What it does NOT change is the ratio of light, so if your subjects are darker than the background, they will remain that way.
Flash is useful to brighten the foreground which is useful for back-lit situations. It falls off with distance, so flash as little to no effect on background which is why so many flash photos look like they were taken in a dark cave.