Switch to user view
Sometimes you have very granular rights for users in place or bug reports which need to be tested from the users perspective. Since you don't know their password you have to copy their groups into a test user first and log-in with the test user. This can be very time consuming. In other tools you can often switch into the users view without leaving your current session (and often also switch back). I would prefer a role/rights object for this.
Johannes Müller commented
Even a basic implementation could be of much value - e.g. using a special syntax for the login (Web+Excel). Assuming you are using the "admin" user with password "admin_password" and want to test "userA":
One could use a delimiter that is probably forbidden in user names (especially the @ should not be used in case of user names = emails). Anyways users who would want to bypass authentication would need a special role.