(1) Microsoft is trying to expand their gaming division, but struggle with first-party games. This acquisition is an acknowledgement that MSFT needs Bethesda creatives.
(2) Microsoft's big strategy right now is to build their Xbox ecosystem - they're pushing GamePass, Xcloud, etc. heavily, and trying to become Netflix for games.
I'd guess they're basically buying Bethesda's key franchises to drive GamePass subs. They'll build them quick, lock you in with a $10/month sub, and let Bethesda slowly merge with the mothership.
Short term, I'm excited because I want these new games! Long term, I fully expect Bethesda to get hollowed out.