The ease of use when creating new users/groups etcThe ability to assign emails as well! You can also set up rules that are pretty easy but could be made more intuitive - like allowing only one email per group or something along those lines? I think it's great because you don't have access logs everywhere in order make sure everyone had accessed any file stored within their folder; so this helps alot since we just log into our account once every month instead having an admin do all these things manually.
It makes managing groups easier too which was my main issue before i started using arra (since then everything has been smooth sailing).