Jomsocial has it's own user profile system and registration process. But this system does not allow you to set the registrants in different Joomla usergroups. Meaning that you can not take advantage of the core ACL functionality of Joomla for other modules, categories or content items.
The purpose of the plugin is to set each registrant in Joomla's usergroups based on the Jomsocial Profile chosen. You can assign any or multiple usergroups in the backend which should be added. Joomla Usergroup Mapper (or synchronisation) is a very small plugin which will work silently for you in the backend.