A plain text file doesn't really do it when you are managing a large number of passwords. Equally, it is not possible to be online at all the times that you need a password (datacentres will often ask that do not take your mobile phone into secure areas). Besides, a solution that requires an online connection introduces a level of risk that isn't acceptable - the last thing you need is to tell a customer you cannot get their systems back online is because your phone ran out of juice as you were getting the admin credentials