The best feature (at least for me) is that you can whitelist some specific websites to always open in a new temporary container, without any extra effort, while leaving the rest of your websites like they are today.
For example, I have YouTube (and any other Google website) to always open on a temporary container but other sites like HN and Reddit are on a normal and permanent container.
I believe Incognito disables (some?) addons, and it has a collection of settings which may mean it acts differently from regular browsing (Tracking Protection is probably the biggest, although I turned that on for normal browsing too).
It's also unclear to me where one Incognito session stops and the next starts, although I'd assume they live and die with their window.
More importantly, Incognito opens a new window while Temporary Containers open in the same window with coloured tabs. So you can have _lots_ in the same window if you want.
My understanding is that First Party Isolation maintains _separate_ caches per first party, but I don't know whether temporary (or multi-account- for that matter) containers also do that. It seems like it would be reasonable for them to do so but I can't find a reference.
That gets you a clean container every time you click the button. Very useful for development testing, too :).