Why does this toilet valve have corrosion on it?
I cannot answer whether it needs replacing, but I can tell you why this happens.

It happens because it gets cold and water condenses on it.

Whenever you flush your toilet, cold water runs through and cools it off. Then the hot, humid air around it condenses water on it, making it moist. Because this happens so often, it will be moist a lot of the time, and thus corrodes.

Why this doesn't happen other places: The toilet is the one thing that uses most cold water. Your shower and sink uses more warm water, and not as much, and thus aren't as exposed to this phenomenon.

Unless it leaks I think you can leave it on.