It's a button to manage your Microsoft account that you signed into windows with. It's not shocking that it has an option to upgrade said account

Edit:

1. I don't use windows, stop assuming things about me
2. Everything you don't like or don't use isn't an ad. This is no different than Gmail and Google chrome having Google drive integration.
3. Microsoft has been forcing people to use online accounts for windows login, so it's only natural for the account button to let you manage it as well.
4. This isn't an endorsement or a value judgement.