The "Airdrop" is a coin/token distribution method, where you (the investor) has to to do one of the following things:
(1) Own the currency that is the target of a hard for, e.g., Bitcoin Cash was airdropped to all owners of Bitcoin near the end of 2017, OR
(2) Sign up for a new cryptocurrency's notification distribution model, e.g., the new crypto currency is giving away free coins as part of their initial coin offering
The latter is more risky than the hard fork approach, for the investor. If a new cryptocurrency needs to give their coins away for free (and is not involved in a hard fork attempt) their business model for the coin/token must be very, very weak. If they had a better business model people would want to participate in their ICO.
Why do you think that every business model with free tokens (airdrop) is weak? It's just one of many ways to advertise your project. Yes, most people will just come for free tokens, but they will spread a word about the project and maybe some investor will hear about it.