I’m always 10 years late to the party but just starting using Containers this year.
Use Case: Librewolf. It’s my Email/Commerce Browser. Bunch of accounts that can’t see each other in theory.
Conventional Wisdom: none of my accounts can see each other, ever
Dissenting Voices: all the cookies chat with each other in the Mozilla Cookie Toilet or something
this, mind you, is cursory research only..
Have trust in those little nosy bytes. They can talk even at Starbucks or Walmart. 
Using LinuxMint I had some webapps to access different sites. The sites can’t see the cookies from other sites, your browser and your system can see everything. That’s called life, someone sees and knows more than others.
For managing different use cases, I use different profiles in firefox, like shopping, vacation, biking, private, business… Let me have different bookmarks and cookies, fits for my needs. At the end I made a ramdisk for my mozilla cache and start firefox with firefox -p to choose the profile at start. Workflows come and go.
I set this up some time ago, with auto-opening rules, so until I saw your post had sort of forgotten it is not normal (or necessarily useful). As I understood, its less needed on librewolf between different websites/domains because of their stricter settings (see here on their FAQ). Didn’t stop me setting it up on reflex when I moved from firefox, which I did when the amount of kruft from mozilla I want to disable on firefox exceeded number of rules I like to relax slightly on librewolf.
The actual “non habit” use case I see remaining for containers is when you want different instances of the same website, also confirmed in the FAQ linked above. That can be handy to get around dynamic pricing, without losing any history via incognito or switching browsers.
Sure, it’s a handy feature. It’s very notable on e.g. shopping sides, so can look up things in peace without the search history following you around forever.
The stricter isolation container tabs provided once is now being handled by setting “Enhanced Tracking Protection” to “Strict”.
These days they aren’t as useful anymore on their own. What they do still provide is the ability to login to the same site with different accounts if you set up different containers for that site.
^ that is everything. might be redundant on lwolf as I am in strict settings—firefox kruft? I was such an about:config under the hood expert mechanic I should have been charging by the hour. Hard work to keep up with anymore on FF and github ‘profiles’ were too lax for me…
I appreciate them for that
I figured between three ‘purposed’ browsers, containers would serve the interests best of my ‘commerce’ browser
this is interesting. when I have to fly and buy tickets I can’t seem to shake the sameness of (the already horrid prices) between browsers…it’s like Airlines have unholy black magic price fixing going on solely I am guessing, based on your IP address------I never thought to fake them out (5 instances of Southwest, for example) between containers. I must try this 
someday I will be that savvy. I want to be
Containers are pretty good at isolating stuff between websites, especially when cookies and other browser storage is considered. They are not excellent, as browser fingerprinting, including ISP IP identification is a big issue and cannot be solved by isolating only cookies and other browser storage mechanism.
Having said that, it is still best to use them and would recommend users to use them.
Never! I don’t like to be contained. 
I’ve been using containers and temporary containers with Arkenfox for years. Perfectly set up, no problems.