In the old days, users used a platform only once and suddenly an airdrop was announced. That happened in the first one, the Uniswap airdrop.
Airdrop appeared because projects needed to distribute their tokens to participants, and the sale of these tokens could subject the project to a fine in the future. I still see a lot of people who are hunting for airdrops, but I only follow large projects where good airdrops are possible: StarkNet, layer zero, zksync