It can get pretty annoying to have to re-login to certain websites every time you visit them in a private browsing tab, whether you’re doing it for testing something or to keep certain activity off of your browsing history.
Much of the annoyance comes from the fact that autofilling saved logins is entirely disabled in private browsing mode, requiring you to manually enter your username and password every time.
I think it would be very useful to be able to flip a switch and allow autofilling certain logins in private tabs, cutting down on the time spent logging in.