I do not generally mess around with cheats or command blocks, but just recently, while I was tinkering around with cheats, I turned on flight while I was in survival, and it was awesome. Not only did I not take fall damage, but I was able to more easily get in and out of my quarry, and I was able to generally get around my world MUCH quicker. But my main use for it is building tall structures. Building tall buildings can be extremely dangerous in survival mode if you do not have scaffolding, and even if you do, it still presents an element of risk. And flight REALLY changes combat. It enables you to move much more quickly, get above your opponent, and, if you need to, it allows you to make a quick getaway.