That turns out not to be the whole story.

Hiding the technique to compute the hash is relying on security by obscurity. Ideally you want a system that is secure even if potential attackers know what methods you're using.

It doesn't add to the security, but I might find it easier to remember 20 MB of redundant and meaningful stuff than to remember 384 bits of literally random stuff. The entropy might be the same, but my memory is not a computer. I can remember vast amounts of material that is meaningful and use it as a password. I can't remember 384 bits that have no meaning.

The benefit isn't in the entropy, it's in the abilities of your users to remember their passwords/passphrases in the first place.