This is one of the things I used to like IE for - perhaps the only thing it got better than the competition:
File | New Session
... and then you have a fresh session context that doesn't share cookies with the others so you can log into services as different users. As a developer this is very handy for testing multi-user workflows, and had numerous uses as a more general user too, and it was easy to do ad-hoc without needing to setup multiple profiles first.
It didn't work for apps that are sensitive to permanent cookies (and other client-side storage) instead if session level options of course, where multiple profiles does (as presumably does Firefoxes containers? - I don't know as I've not yet used them).
That does sound useful. I often end up creating several temporary containers while doing testing and development. The overhead of creating and deleting these is annoying.
Yes, but you can have as many active sessions as you need instead of just two.
Incognito/InPrivate/whet-ever-other-browsers-call-the-feature tabs/windows share the same session so that gives you at most two active sessions: normal and incognito.
The "new session" option in IE can have many more. For a lot of workflows two is enough, but sometimes I want something like "a couple of distinct base users, a manager, and an admin" for testing more complex user workflows.
I don't know how edge does incognito but in other browsers, incognito kills the tab history, so if you accidentally close a tab you can't trivially restore it with ctrl+shift+t or whatever.
It didn't work for apps that are sensitive to permanent cookies (and other client-side storage) instead if session level options of course, where multiple profiles does (as presumably does Firefoxes containers? - I don't know as I've not yet used them).