By default guest are considered as people just visiting the site. In the admin panel/ users / user roles they have limited capability. If they want to interact this entices them to become a member whether it be paid, or free. The limitations helps keep just anyone from entering your site, and interacting with your members, or posting things you really don't want. If they become a member, and interact in an inappropriate way, you can at least boot them off. You can't really do that with guests as they have no profile. I have my site closed to registration by invite only.