its free and they can download it right now,what do they need steam for?i dont get it
Steam acts as a distribution platform. By advertising the game there, it would heavily increase the game's potential player-base . That said, new titles don't usually stay on the front page very long, so strategically it would be best to either fully release the game or offer a paid "early access" version of the game.
Another benefit I'm hoping StarMade will make use of is the Steam Workshop. Giving players a wider selection of locations to upload/download content ensures that more people are likely to keep playing the game. It's like how you don't want to use forums or a website as a primary source of information because only a small percentage of players will care enough to search there.
Personally, I don't think there's any urgency to get the game on Steam. I'd only advocate for a Steam release if the development was seriously in need of funding and hopefully that will never be the case.
EDIT: Just wanted to add that even in the game's free state, a lot of consumers find it convenient to be able to keep all their games local to a single platform like Steam. More options, more success.