The main goal is money, an Xbox branded windows PC has potential to drive sales.

Microsoft can also hopefully target a smoother user experience than a typical windows PC provides. They want this to be a valid console competitor, but just slapping xbox brand on a windows PC isn't enough to do that.

Having a first party hardware device to target for PC games can also help devs with having a clear performance target for PCs, similar to how the Steam Deck is currently a minimum spec performance target for a lot of games.