Two reasons I can think of to force the creation of a MS account is to a) make it easier to get started using MS products (e.g. Office), since you don't need t

Two reasons I can think of to force the creation of a MS account is to a) make it easier to get started using MS products (e.g. Office), since you don't need to go through the hassle of creating an account; it just works and b) once (almost) everyone has a MS account, using MS oauth may become the default login mechanism for many third party services (although Google is already starting to take up this space).