Hi there! To answer your first question, it's any of the above :) If you use Hotmail, Outlook.com, Xbox Live, etc. they all are associated with a Microsoft Account. You can also associate your existing e-mail address, which you regularly use with a Microsoft Account, if you prefer that.

As for why, it's really a good way for us to stay in touch with you and keep you updated about what's new, help you get started, and make you successful in general. Visual Studio Community on Windows has the same behavior for the same reasons. Hope that helps!