Why would people launch them?
Marketing, attracting other investors and giving their project attention that they want.
If there is no airdrop then they have to pay for other advertising cost.
But if the project doesn't gathered that much fund during the ICO then that airdrop is worthless.
Devs do airdrops before/after ICO? I'd guess ICO investors are not going to be happy about that as airdropped tokens would be dumped below ICO price as soon as the token is listed on an exchange.
They are doing it before the ICO and they are distributing those free tokens after the ICO.
There are investors that doesn't like those ICO's that does have a big allocation for bounties, airdrops and marketing.
Because after getting the rewards, most of those bounty hunters and airdrop claimers will just dump those coins.