Airdrops are timed Gold deposits that are distributed to players at regular intervals. They are funded by through in-game transactions, a portion of which is collected into a public pool and redistributed to all players with active civilizations on the hextile Earth.
Airdrops provide a means of passively generating ETH. since Gold is exchangeable for ETH at the Bank*. This regular flow of currency promotes different play styles, especially defensive ones (See Guardian) and ensures that all players have Gold for transactions.
*Pending regulatory status in your location.