This is not something you want to use all the time, but it is very useful when trying to determine the experience of a particular user. With all the capabilities available with the ACL it is useful to be able to log in as the user to see what they see. This plugin lets you do that without having to set/reset the users password or ask them for it.
Please note: this plugin can allow an admin to masquerade as another user which is a recipe for abuse. Hopefully only ethical admins will install it.