They usually do it to generate attention for their project, giving a small amount ( 1-5%) of the total supply away for a lot of attention (which could result in potential buyers) is probably profitable.

Some airdrops require you to have another digital currency, so you could argue that alot of people will buy that currency just to participate in the airdrop ( which could also potentially result in a lot of money/higher marketcap).

Or sometimes you need to complete offers to receive your share of the airdrop ( sharing stuff of them on social media, complete certain offers etc..)