"the point of a good (cryptographic) message digest is to decorrelate patterns in the input from those in the output hash."
The point of a good cryptographic message digest (hash) is (amongst others) to simplify detection of whether a message has been tampered with in transit.
If the hashes are different, the message has been tampered with. 100% confidence.
If the hashes are identical, the message may still have been tampered with. You don't *know* either way unless you refer to the source, which is in many cases too tedious to contemplate.
Same goes for checksums and retries in TCP etc.
If the checksums are different, the messages are different (the received data does not match the transmitted data). 100% confidence.
If the checksums match, the received data still may not match the transmitted data, but there is a level of confidence which is greater than zero (and significantly less than 100%).