Comment by WorldMaker

They are definitely orthogonal use cases. I find I prefer Profiles for different accounts/identities more than Containers (including profiles being very different windows of Firefox reducing confusion of which identity I'm looking at). Containers I like using for those hard privacy walls of pinned sites to avoid more "personalized" ads. But the flexibility exists to mix and match and use Containers and Profiles for either/both use cases. I don't think there's a "wrong way" to use them, just your personal preferred way.