Comment by reciprocity

I also think the claim that "the brain does not retain information it does not need" is an insufficient explanation, and short-sighted. As an example, reading books informs and shapes our thinking, and while people may not immediately recall a book that they read some time ago, I've had conversations where I remembered that I had read a particular passage (sentence, phrase, idea) and referred to it in the conversation.

People do stuff like that all the time, bringing up past memories in spontaneity. The brain absolutely does remember things it "doesn't need".