For Confluence administrators, grouping users in Confluence is a great way to cut down the work required when managing permissions and restrictions. Groups are also very useful, however, to anyone who's a space admin, or can apply page restrictions.
If you're a space admin, you can assign a set of space permissions to a group rather than to each individual user. And as a page creator with 'Add/Delete Restrictions' permission, you can also add and remove page restrictions for groups.