Quoting from base-passwd again:
Allows users to add local modifications to the system (/usr/local, /home)
without needing root privileges. Compare with group 'adm', which is more
related to monitoring/security.
Note that the ability to modify /usr/local is effectively equivalent to
root access (since /usr/local is intentionally on search paths ahead of /
usr), and so you should only add trusted users to this group. Be careful in
environments using NFS since acquiring another non-root user's privileges
is often easier in such environments.
... so in practice, staff is root-equivalent, but in principle it's not meant
to be. (Yay.)