Flying is the cleanest evasion in the game: a creature with it can only be blocked by other creatures with flying or with reach. In practice this means most ground bodies, which is to say most creatures, simply cannot stop it. Damage gets through. A two-power flier and a two-power vanilla two-drop look alike on paper and play nothing alike, because one of them is going to connect ten times over the course of a game and the other is going to run into a 1/3 on turn three and stay there.
This rewrites combat in two ways. For the attacker, flying turns a modest body into a clock: you can size the threat just large enough to dodge the format's common air defense and ignore everything else. For the defender, it forces a separate axis of answers. A board full of ground blockers is worth nothing against a 1/1 flier holding an equipment, and decks that ignore the sky tend to lose to creatures they could have killed with any removal spell they declined to draw. Flying is also why so much efficient removal in white and blue is templated to hit fliers specifically.
The ceiling is the body. Flying on a 1/1 with no support is a chip-damage creature and not much more; flying on a finisher is often the entire reason the finisher wins. The keyword does not protect against removal, does not break through a mirror of other fliers, and does nothing the turn it is cast. It is evasion, not advantage, and it is only as good as what it is attached to.