In my experience, flight breaks down into three tiers:
- The party has no flight capacity at all.
- The party is mostly ground-bound, but some portion of it can fly.
- The entire party can fly.
Each tier dramatically expands the PCs' capabilities, which in turn closes off a bunch of options for the DM in adventure design. Going to tier 2 opens up room for aerial scouting, sending a flyer to tie a rope or retrieve an object, and "air support" in combat. Going to tier 3 allows the party to soar merrily over all manner of obstacles and threats, and demolish melee enemies at no risk to themselves.
BUT:
The find familiar
spell means tier 2 is already available from the start. It doesn't actually make much difference whether the flying thing is a PC. An owl or hawk familiar allows the party to reap most of the benefits of tier 2 right there. When you hit third level, warlocks get Pact of the Chain, beast master rangers can call flying companions, and so forth.
If it weren't so easy for a low-level party to push into tier 2 already, I would object strongly to "flight at level 1" races. However, that ship has sailed. Aarakocra and fairies are just another way to get to the same place as a wizard with an owl or a beast master with an eagle.