Groups give builders the ability to clump things together.
Groups are extremely versatile and can help:
keep tabs on what players have done/failed to do,
what guilds players have joined,
what type of person the player has chosen to be,
what liabilities special abilities have been acquired
Groups can be used as flags that can be set or reset in ways
controlled by the people controlling the group.
Actions can be hidden or revealed, permitted or denied,
depending on what group memberships a player has acquired.