When an ICO is announced, cryptos fall in dire need of marketing. They need thousands of users to make the crypto world notice them.

So instead of saying"here I am" they offer a little piece of what they are offering for free.

They basically offer tokens if you make their user base look heavy
eg; they"ll tell you to join their twitter, telegram or subscribe to their official page with your email etc.

If the airdrop succeeds and their token gets registered into a viable and renowned exchange platform like poloniex or bittrex you will be able to trade your coins there for real money AKA fiat cash or other crypto tokens. example: Byteball.

Join and collect as many airdrops as possible. but do little research beforehand like if they have a solid project or not, who is the founder and if he or they had previous successful icos. Who knows you might become a part of another bitcoin.