I imagine the following scenario:
One has a side, e.g. for a newspaper. You want people to submit
events, parties etc. So you want everybody, even anonymous to be
able to use the calendar.
On the other hand you have events that are just internal, thus you
want them not to be visible to everybody, but just a certain-user
So if one would have groups of events and could assign these
groups to different user-groups it would allow the calendar to be
used for public as well as non public events.
Another possibility to realise this would be to be able to have
several calendars, which you can already, and assign these calendars
to certain groups. Anonymous could then e.g. post in parties and
see parties, whereas the calendar internal-events is only visible to
editors. This way one could display two calendars for editors, i.e.
one internal and the normal/public one.