It's essentially a way for a coin to immediately "create" a market from thin air. They give everyone a piece, find an exchange to list the coin, most of them also have the community develop parts of the project.

Free money, an instantly created market and community built around it. Sounds too good to be true and it probably is, but might as well gather them in your free time, nothing to lose right? Some of them end up paying off in the end you just either have to get in on the first couple of rounds, or get lucky with a couple of them by holding everything you accumulate.