Group Calendar Period/Event are used to defined the (un)availability of a Group of person, which means there working time period.
The group of person is defined by associating the person's assignment to the Group Calendar through the calendar base calendar.
Except this, it works as personal calendar.
2 Period portal types are required, because the user interface may change (ex: to restrict the calendar_period_type categories). Another reason is that quantity may be calculated automatically (from start and stop dates values), and in one case, it has to be a positive value, and in the other case, a negative value.