This is an example of the Registration Entrance Page Type page. It provide two functions 1) to login an existing guest User (e-commerce customer, etc.), or 2) to register a visitor (and create a user account for them).
The advantages of registering a visitor is that they will have access to the My Account features, so they can manage their own contact information, address book, e-mail preferences, view saved carts, and reorder. In addition, they can then take advantage of all pre-fill form features.
You can make registration optional for any Section of the Site Tree (which is used for e-commerce customers that might only order once from your website and don't want to register).
TIP: You don't have to link to this page. Simply linking to a protected page (or file) directly and let the system prompt the user with this page as necessary. You should only have one page in the Site Tree that has this Page Type and it needs to remain in a Public Section.