I will gloss coat for all of the above reasons, especially the protection one, it also gives a little extra deepness and blends the colors, something to do with light difraction. I will also gloss coat when I paint a figure that I am not going to use for a while and that will be out in the open as opposed to a sealed display case. the reason being that dust is sometimes very hard to get off of a matte finish, but with a gloss finish just a quick swipe or two from a big soft bristled brush and its all claen and ready for the flat spray before the game.
Some people used to gloss metal, leather, eyes, fluids etc, they would then flat finish clothes and all fabrics except silk, and then they would mix gloss and flat to get an eggshell finish that they would apply to the skin. It usually looked pretty good. steveb