To play devil's advocate, while I have little doubts that these specific ones are scammy, there are many cases where they have been genuinely good value. In general, this is when a product requires an established user base in order to be effective , so they'll have an airdrop shortly after opening and then another a few months into the product's life. The aim being that if X number of people try it a bit, hopefully a proportion will continue using it and make the whole product viable.