As mentioned in the documentation:
Group Calendar must be enabled to have all the Calendar functionality. When Group Calendar is not checked, the only Calendar feature is the ability to create personal appointments and accept invitations to meetings,
also, users cannot share calendars.

What does this feature do ?
After deactivating it for a user, I couldn't see a difference in calendar behaviour.
even the shared calendars still worked (perhaps because they were added before disabling the feature)