Those are for different things. I use containers to log in to the same website with different accounts without having to switch browsers or open a new window. E.g., since Discord still doesn't support switching accounts in the year of our lord 2021, I have a container set up for the discord account I use for IRL stuff, and my personal account logged into the "vanilla" container. I can keep both discord tabs open side by side and get notifications from both.

Even for websites that allow switching accounts, I prefer to use containers especially if the accounts go across different "domains" of my life. If I must log in to something personal at work or vice versa, I generally use a container for that to keep them separate.

I use containers to only use some google services while signed in. For instance I use gmail signed in but I google search in a separate container to not log that data in my account.

I may be fooling myself and google can identify it anyway through my IP or something but it feels like I get a little extra anonymity.

Earlier this year when PS5's were really hard to get, I used Firefox Containers to help get one. One of the tracking services would tweet that some retailer had them for sale. I would use containers to open a dozen connections to the server. A couple containers would get through and eventually I was able to complete the purchase transaction.

It took about a week of trying and I was successful at Walmart and BestBuy.

Containers are necessary to provide account segmentation.

I wrote up a quick email to friends/family at the beginning of the pandemic about Firefox containers in regards to the vaccine signups that were occurring. Since there was site isolation you could get as many “slots” as you were willing to solve capchas. Most of my friends and family were vaccinated because of this feature. A bunch say they use it now a days for normal site isolation but I did at least convert some people off chrome that way.