Have you ever gone to a supermarket where there's a hotdog stall that offers free taste. That's also the same with these airdrops, they are attracting people all in all by giving such tokens for free.
That makes these airdrop people announce and tell others that they've found a good token which results them to advertise that project.
Pretty much this explains the purpose of airdrops. Usually, I can see many coins giving just 10 or 20 coins that may be not worth that much but many are participating in it because it is free. You just have to join telegram community, like their page and follow it as well with sharing to others about the airdrop. When they airdrop to 10,000 people and share it let's say, 1,000 friends or followers per person then 10 million people would be reach because of that airdrop and a possibility of catching an investors eye.
Of course bounties are still better because there is constant sharing of information and being active in community as well but airdrops are still an effective tool for me as well to get exposure and get people to share about ICO if they don't want to participate in bounties since airdrops are just usually one time tasks.