Well I was approaching it from the point of view that you verify the image is correct and then you guarantee you'll always use that image and not some other version given that tags are mutable.
Can't the hash be verified by the client too?
Sure, but who tells the client what the correct hash is?
If you're the entity who created the image you can retain the original hash and verify it against the downloaded copies. But that kind of defeats the purpose of being able to download docker images across distributed hosts.
They'd really need to be signatures attached to the images, not just hashes.