apart from what @Davros said, some people might decide to buy the game if they like it. In addition, some other people buy DLC (I purchased some DLC for Alien Isolation in the gamepass PC version for instance).Does the amount of players on GamePass benefit them financially?
Also, this could help sales of the franchise or related games. For instance, I had F1 2015 and 2017. After playing F1 2018 on gamepass PC I decided to buy F1 2020 day one on PC (steam).