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 is the perfect answer I guess. It's the fastest way to spread about the project to build a strong community at the same time it does not cost them much as they usually pay the airdrop participants a portion of the total supply. Same with bounty hunting, to help advertise about the project.