it seems there's no reason to be a patreon ,except doing charity just because you want to give them money in exchange of nothing.
That is exactly what Patreon is intended to be. Patreon is NOT a marketplace where you buy stuff. Patreon is derived from the word "patron", a person who supports another (often financially). Quite common in the phrase "a patron of the arts". Patreon is NOT about paying money to purchase a product. It is about supporting artists/content creators in order to enable them to do what they do. So yes, in essence it is a charity. Usually the Patreon owners attach exclusive perks to that, which is only fair.
Obviously the patrons don't get anything additional out of a Steam version. Steam is just a way to bring the game to an actual marketplace where people can, you know, BUY the game. Both platforms are independent. In essence, a Steam version is precisely for people with your mindset: "I pay x amount of money and in exchange I am entitled to Y product". That is not how patreon functions.
The reason the Steam news is on patreon is that they use that platform as a general news outlet. You may have noticed that the Steam post on patreon is public. It is not intended for patrons only, but for everybody who might stumble across the page. And for those people it is relevant information.