Some airdrops are just scam because they just want to get the information of some people. When you fill up the form, some require the information of private key and for those people who doesn't know the importance of private key, they easily get swayed and give it out only to find out later that their coins are already gone. There are some airdrops that are legit and may have some value. You just have to wait for sometime for the coin to have some value because many of them are late bloomers and some are being hype but with no real value but you can still benefit from when you sell at the right time.