Groups are pretty useful. Even though they are not exactly the same as containers (on Firefox you can have multiple containers in one group using the extension “simple tab groups”), it would make groups actually useful.
All Containers do is isolate Cookies, so that you can login to multiple accounts.
This is also useful for privacy: You dont log in while browsing Reddit, Github, e.g. but if you want, you open a Container where you are logged in, and post something.
Containers are one of the only things making people stay with Firefox.