Are blocks hashed only to slow verification thus reward release rates? If not, why?
No. The block hashes link the blocks together into a chain. The hash of the previous block is included in the data that forms the next block and therefore contributes to the hash of the next block. This, combined with the proof of work part of the algorithm, causes the chain of blocks to act as a kind of distributed time-stamping service that proves when (and that) each bitcoin transaction took place. See the 'Proof-of-Work' section of the Satoshi White Paper for more details.
