Hiya!
There are several projects in the market offering airdrop or bounty. This is because projects are in an early stage and they need members or traction to go forward. Another alternative for companies would be traditional marketing. So airdrops are basically marketing funds used to get attention - and it works nicely because it creates a win-win-situation
There is no investment required for airdrops, if someone requires you to invest before you get your airdrop it is most likely a scam and I would recommend you to stay away from them. Anything that sounds too good to be true, usually isn't true. So be careful out there!
Airdrops are a good way to get to know the project, you might find a good one you want to invest later. Pay attention to the business logic, the culture and the people in the community - they will tell a lot about the future of the project.