If you used a previous version of Moodle and enrolled students in courses using the category roles, activating this enrollment method will enable you to import those enrollments into the new Moodle. If you're not importing category roles from a previous version, you can leave this enrollment method deactivated.

In MoodleCloud and Moodle on-premise, an easy way to manage your enrollments (including ones using PayPal, and such) is found using the following path:

Administration | Site administration | Plugin | Enrolments | Manage enrol plugins