Should the game raise a lot of money, a future Steam version could be released thanks to the backers who purchased the game.
However, this depends on how much money is raised.
For everyone who has already purchased the game on Itch.io, the DLCs or bundles will be sent to you as keys via email if a Steam version is released, so that you, as backers, don't have to pay for them again on Steam.
This is as a thank you, should the goal of reaching a certain amount (I won't mention any specific figures here) actually be achieved.