Most of free tokens, especially got from airdrops from some altcoins are have purposes; probably their advertisements which people will start to check their project or what the token uses.
And most of the free tokens are from random projects that are requiring you to do particular things, like subscribing to their social media accounts, like twitter or subscribe to their newsletters, most of these are called airdrops, but it should not be an airdrop once they will ask you for something to do.

Just be careful with those free tokens, probably they are scams and spams only.