They do this and keep their top selling games exclusive to their platforms. Games like GTA have shown just how much money you can make off micro transactions. No company is going to just jump out of that market. They use their top games so they are more liley to get people to just have it installed. That way you install the launcher, put in your credit card, and you are only a click away from handing them some money.

There can be positive benefits though, auto updates, update news, community messaging or content.

But mostly so they can get the hardest part of selling done, a valid payment method input.