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.