Specifically it allows access to some areas you wouldn't otherwise be able to reach (Arial City as an example) and means you don't have to have a boat to cross water (but you always should anyway).

Not being able to be tripped is a huge deal though. Every melee class gets trip, and two of them can trip you to death pretty easy if you can't fly. Trip is also pretty reliable, so it's an easy way to get ganged down in a way that you can't do much about.