Provides to admin users a way to select what categorie determined users can write. (administrators have access to all categories)
All the preferences are deleted for provide a best performance of wordpress. So, if you remove this plugin, all your preferences goes away.
Administrator have access to all preferences on your blog, this inclues the administration of user-cats-manager, so hano no sense it have restrict access. If the users are setted as Administrator, it can write on any categorie. So, for new users choose editor mode.
You can read more about in http://codex.wordpress.org/Roles_and_Capabilities
You can use too the Role Manager to have more control of your users. http://www.im-web-gefunden.de/wordpress-plugins/role-manager/