Deleting a group in Admin → Groups does two things beyond removing the row.
Its members are moved into the default group. A member in no group at all has no permissions and no way back — they simply stop being able to use the site, with nothing to explain why.
🚨 Every restriction pointing at the group is cleared too. This is the part that matters: a page restricted to a group that no longer exists is a page restricted to nobody, silently, and you find out when somebody reports it missing.
Some groups cannot be deleted at all — the Guests group, the group new members join, and the last group that can administer the site. Convoro refuses rather than letting you lock everyone out.