All of them simply ask for your ETH address and want you to join their channel on telegram. I mean, it doesnt make any sense.
Why would people buy when a lot of people are getting them for free.
You're right, most of them are useless trash and cost nothing. That's one of the reasons why they're on airdrop: the creators are trying to get more attention to their coin. Also that might stimulate people to buy the coin.
Also they're doing that for getting more people on the Telegram channel so it would look like their project is a good one and popular one so it would look more reliable and trustworthy.