I'm distinguishing between flying and hovering. A hovering creature has fewer concerns than a creature that has to fly at an appreciable speed in order to avoid falling.
Different available attacks
A large dragon has a long tail that it could use to whip players. Using it while flying is difficult. Similarly, a creature with prehensile wings might be able to grapple or slice at enemies while not using them to fly.
A breath attack from a flying creature affects an area in a straight line. A breath attack from a creature on the ground affects an area in an arc. If it is difficult to line up your targets for a bombing run, as it were, it might be easier to get a good concave.
Even assuming a straight line is desired, it's going to take some finesse and timing and cooperative terrain to line up four soldiers on the ground for one flame breath. It could be easier in some circumstances to land.
Landing allows a flying creature to dish out more attacks per minute than remaining in the air. You don't lose time wheeling around and lining up your next shot. This is especially true if you want to concentrate on a single target. Easier to land and pin them down with your talons than to try picking the person out of the crowd and bank just right to be able to make one swipe as you pass and then wheel again for another attack ten seconds in the future.
A flying or hovering creature with a soft underbelly can land and have their weak points protected by merit of being too low to the ground for their enemies to attack.
If you want to keep people away from a particular spot, the easiest way is to park yourself on that spot. If you're hovering over, that's decent for zoning, though landing is better. If you're flying, it's bloody near impossible to zone people away from a spot and still get some meaningful damage in.