The big moneymaker in Xbox has always been content, not the boxes. At first, those were even sold at a loss. In this scenario, Xbox just gives MS another box on which to stream whatever content they offer -- and one that is not controlled by Roku or Samsung.
It could be argued that "because it makes money" is the only valid reason to go into a business. Certainly Microsoft can afford to buy any content at all that comes to market, just like Amazon and Netflix do. There's no special recipe here and in the age of cord cutting, consumers are extremely fickle about where they stream.
MS also has the unique experience of vast profits from Xbox Game Pass, another decision which has costs them millions in content acquisition (by buying studios) yet returned many more millions as profits from recurring monthly subscriptions. Sound familiar? This is Netflix's model, too, MS has shown they can already play that game profitably.