Especially, when configuring stuff that others will acutally use or to understand an error reported, it comes in handy if one could actually _become_ the user in question to see exactly what she sees based on her permissions and settings. (One can of course set up a nubmer of dummy users for this during installation and peek over her sholder in production.) Therefore this proposes a kind of `su - ` functionality for Koha on the web. Usually a function like this will required pretty high administrative rights and it also comes with some implications wrt to privacy. In real life I usually use such a function pretty rarely and usually with the patron on the phone. "I do not really understand what you are seing right now. Mind if I _become_ you to see it as you do?" And there are even some regulations when I am allowed to do this. Still during setup of a new system especially if you are new to Koha.
I agree this could be useful. I also agree it has big privacy implications and needs to be a feature that can be easily disabled system-wide and have its own specific permission when in use.