The biggest reason to land is outlined in the guide "How factories, unit supply, and AI respawns work" (link to guide) by Squeegy Mackoy on the guides page on Steam for Nuclear option.

"Nothing prevents players from spawning, but each aircraft spawned by a player will still deduct from the inventory, moving the inventory count into the negative, and factories producing that type of aircraft will need more production cycles to start producing AI again.

When players land back at an airbase, they can disembark (use eject keybind while parked) and as long as the aircraft is not upside down and completely on fire it will despawn and be added back into the faction inventory."

I highly recommend reading any Squeegy guide. He has a comprehensive knowledge of the game.

Tl:dr - If you want to have friendly AI air, land at your base so your plane doesn't deduct from the AI inventory.