The way I see it which is now like the latest thing going on around the sphere of Cryptocurrency, airdrops are usually
Done in a bid to publicise a token by way of giving them free tokens. Now a number of time tasks such as retweeting, sharing their posts on Facebook could also be required to qualify for the airdrop. So to cut the long story short airdrops are a way of creating publicity by giving out freebies to the public
Please can someone throw more light on airdrop.... And why do some Cryptocurrency give airdrop