There's different reasons why airdrops can make sense. As a general rule, to have a strong market, yu need as many people involved as possible. Getting a percentage of the tokens out to a lot of people can halp with this. Even if all these people start selling their tokens right away, ideally this provides liquidity in the market.

Also, it might help people get "hooked" as in getting interested in the project, reading more about it and ideally getting involved either directly, or through the market (by buying more tokens).

Another "advantage" (well, it is for the team) is, that suddenly, you have a shill army. Everybody wants you to buy their tokens at the highest price possible, so they are telling you anything to pump the price.