Easier and less predictable mobility. Also if the other dude is grounded you can just go up till you're out of range and they can't do much, and you can't be incapped by being dropped down a hole. Kharn the Betrayer has a couple losses via being thrown off a planet, if he could fly back those wouldn't be losses.
Flight is a good advantage against those with trash range as said above by others but an obvious downside is if the character in question has only flight and little to no ranged attacks. They are still forced to get within range of the opponent otherwise it will be a stalmate. However most characters who can fly usually have decent ranged attacks so this isn't a problem for them.