It will let you open containers that trash cookies on close. You can also set certain URL's to always open in a tmp container which is useful for sites that only let you read x amount of article per month.
[0] https://addons.mozilla.org/en-US/firefox/addon/auto-tab-disc...
My assumption up to then was that each newly opened Incognito tab or window was it's own private session, but this turned out to be wrong - they all share state, as if you had one separate but shared profile called Incognito. This was a little upsetting as I'd been using Incognito for years without realising the data sharing going on.
Temporary Containers does what I'd expected from Incognito. Each new temporary container has its own isolated cookies etc from all the others. So now I open a TC when I want to visit a site without identity or tracking, or to login temporarily with a different account, and don't use Incognito at all.
to provide context, i still use incognito, but i consider incognito mostly about hiding things from myself -- i use it almost exclusively for porn, because i don't want my porn habits in my browser suggestions. temporary tabs still land in the recent urls and such.
What I meant is if you open what you would otherwise open in an incognito window in a container instead, you can restart your browser without those tabs being closed and the respective state being lost.