Microsoft is expanding into other businesses than those core ones (where they dominate anyway, they don't have much more to do). You could say the same of Activision Blizzard acquisition, why they go into games (though they already are)? It's obvious they want to go into entertainment.

Though if they do want to buy a streamer, I doubt it'll be Netflix. My guess is Warner Bros Discovery, wait more for the crisis to get a cheaper price (though you still have the debt so it's expensive but Microsoft can afford it and they feel like spending it seems at the moment, they overpaid for Activision) and get it.

They get very strong IP (DC notably but also Game of Thrones and tons of stuff from Warner over the years), prestige TV with HBO, reality TV stuff with Discovery (we can not like it here, it's popular), video game studios with strong IP in DC (might counter the Marvel focus of Sony) and Harry Potter (the new upcoming game seems destined for success and probably sequels). Sure they don't get as much subscribers at once but still there's more than Netflix here IMO and it's cheaper.