Group audience versus group content visibility
The basic concept is if you don't have any access control on the content specifically then the content would inherit the groups access control. But say you had a private group that you wanted some content visible (but not all) then the access control for the content could be set to allow that to happen.

You can read more in the docs here:
Content visibility:
https://www.drupal.org/node/2014755
Group visibility:
https://www.drupal.org/node/2014751

There are even some pretty diagrams listed in the content for more help visualizing these concepts.