What is the reason behind putting Sign-Up/Log In Popup as separate reusable in Basic Bubble Template? IMHO, it is just past of Header reusable logic.
That’s probably so that it can be used in cases independent of the header.
For example if you want to force a user to sign in to use a page, you can set the login/signup popup to appear conditionally on page load.
Actually by rule of thumb, it makes sense to keep things separate if you see necessary use cases where they may function independently of each other.