Airdrops are meant to gain traders of their altcoins, to encourage someone to register to their website, free advertisement to their platform by means of retweeting or sharing on social medias, to gain thousands of supporters or to enlarge their social community on slack, discord or telegram, etc. Actually, that airdrop is an eye catcher to cryptocurrency's freelancers which as known as bounty hunters.
This. But you should still continue to be wary/vigilant of scam airdrops. There are still a handful of it out there.