It's a quickest and easiest way of distribution, unfortunately almost always connected with SCAM. Basically it's to lure investors to buy after hits exchanges and pump&dump after that.
Be extra careful with giving your details, if you really need make sure your passwords are strong on email and forum passwords can't be the same.
My forum account was hacked within 24h after I participated in AirDrop, that was first and last time I joined AirDrop.
Strongly not recommend.
That's why you need to be very careful on where you sign-up. And if you enter your email, at least have some security features activated so even if they try to hack your email, they need to crack first that security feature.
Sometimes, I just let airdrops passed if my gut feel says that it's a potential scam to collect your data. And because it's the hype nowadays and they know a lot of gullible people here just sign-up without even thinking, they try to lure you with free tokens. Sometimes I saw that a certain individual mistakenly give his eth private key instead of eth address. That's too bad.