That’s just the thing. Brave does site containerization for you already, automatically, without the baggage of manual container management. (And it’s getting better with every release.)
So if you aren’t separating things on the basis of needing multiple logins (different usernames) for a given site, then using Containers by hand is a bunch of unnecessary work in the Brave ecosystem.
Also, as a reminder (I have mentioned this before) – but those who are sticking with Firefox should bear in mind that Containers do not ‘contain’ your extensions. So for instance, if you use something like ‘Honey’ for shopping, this extension also has access to your other, potentially more sensitive containers. In either browser, Firefox or Chrom*-based browsers, you’d need a separate profile to split those out; but Chrom* (including Brave) makes this a bunch easier IMHO.
Not trying to come off like a propagandist or a fanboy here, I was in the same camp initially as well, until I had a firmer grasp of what Brave was already doing for me. And after I committed myself to a several-month period of kicking the tires with Brave I would never go back to the hassle of fiddling with Containers. “Letting go” has been great.