I believe there are around 3 types of overall hashes that we deal with in Bitcoin.

1. block hashes
2. Merkle tree hashes (makes transactions tamper proof by hashing them up to the Merkle root)
3. public-private key hashes (used as part of the UTXO model to generate addresses derived from the public key)

I get the functionality of all hashes other than 1st. Is it only there for reference ?? We could have used block height as block reference and traversal and added nonce in that for our POW requirement.