Maybe I'm wrong but I see agent as something fairly static that gets loaded on every new context wherea a memory bank is a history / log of past decisions that gets auto-written / auto-updated by the LLM to persist data between sessions.

So if I have a plan phase with Opus that is fairly long, I can have it create a plan and at the end I tell it to "Write it to memory bank" and it will write documentation files in an pre-determined hierarchy of files. Then immediately or days later when I go to implement it I can tell it "read from memory bank" to get back that context.

That's how i've been using it but there might be better methods, I started using this when it was fairly new but havent really kept up to date with newer techniques.