Google Help on this topic: https://support.google.com/a/answer/167100
Advantages
- It is easily visible who is a group member, and why.
- When someone is added a member to a childgroup, they become an indirect member of the parent group at once.
Details
Drives
Drives accessible to the parent group are accessible to all child group members.
Posts ( = Emails)
Posts in the parent groups are also posted in the child group, if the
person posting has the appropriate permission in the respective child
groups. Therefore, it is recommended to allow at least organization
members to post in all the child groups.
Events
If the parent group is invited to a calendar event, the members of the child group are invited, if the
person creating the event has the right to see the members of the
respective child groups. Therefore, it is recommended to allow at least
organization members to see the members of all the child groups.