Haven't seen this setting - if it exists, feel free to jump in!
As a super admin - I would like the ability to log-in as or impersonate other users. This would allow me to diagnose issues and ensure settings are correct more easily.
For example - changing the bcc email address for one user - I could log into the others accounts and change that for all rather than 1) having all their passwords 2) going over to their computers to change it for them or 3) relying on them to change it themselves.
Our CMS has this feature and our CRM (Sugar CRM) has this feature and it is very useful.