The publisher (a small indie company) thought it was a good enough game that people who don't use itch or who like storing their games in their steam library would be willing to pay a few dollars for it. I for one appreciate the effort they put into the port and implementing controller support, as people had been asking me for those.
You don't need to get the Steam port if it's not valuable to you, that's perfectly fine. The thought of me giving my work out for free being called scummy by someone who benefited from it and then publicly badmouthed me is pretty funny though.