The only benefit I've seen to having a Microsoft account is that I don't have to remember a cd key anymore if I have to reinstall... other than that, what was it actually used for?

They use machine id, shouldn’t need the key to reactivate on reinstall.

But you can move your key across devices -- just de-register the old machine and register on the new one.

I bought a Windows Pro license a decade ago (maybe for Win7) and I'm still using the same license for Windows 11 on a new PC.