iOS has always tried to simulate zero shutter lag: the camera continuously captures full-resolution images into a ring buffer, and the app goes back in the frame buffer to find the image that corresponds to when you pressed the shutter.
Zero shutter lag is what I love about the iPhone. Compared to my previous Xperia Z5, the pictures on the iPhone are not better (rather worse at bad light conditions), but I never have to wait to take one. The time it can take the Z5 to focus and finally take a picture is annoying.
This sounds like a good way to drain the battery, and it will only work when no flash is required.
Still better than nothing.
Presumably it's quite cheap if you have enough RAM and hardware JPEG acceleration; the camera has to capture a string of images to display on the screen anyway.