Groups are a very powerfull tool to grant/restrict access permissions.

Take the demo as example. You have a list of musicevents sorted into
different categories. For each of these categories you can promote
maintainers, means to create a group of people who takes care of a category.

Once you have assigned a group to a category, only members of this Group
are allowed to submit events into this category. Members of other groups
or users who have general access permission to submit events, will no
longer be able to create events for this category (except administrators and
super administrators).

Together with the edit own events flag in the Settings -> Access tab, Groups are a
powerfull tool to manage your Event database.