As a way to spread awareness to the relevant audience of potential investors and eventual enthusiasts, coin teams will, from time-to-time, do airdrops. An Airdrop takes away the payment bit of this process, instead giving more value towards informing people of their offering, and giving every one the chance to own some tokens. Method of distributing tokens is a better way to accomplish the same objectives of an ICO.
Airdrops are usually one time tasks so after the participant receive it, he would just usually forget about it - well, maybe majority of people will but then even if there is one investor in 100 then the team can already add it to their funds. Airdrop is a good way of exposing the projects since most probably, there would be more participants in an airdrop than a bounty but I think bounty is more effective because there is a consistency in sharing of information.