Proof of work has several different roles in a blockchain protocol. On the one hand, it guarantees that old values of the chain can not be hacked, they would be needed to recalculate the whole blockchain starting from the modified block. On the other hand it is essential to the fork resolution strategy, the longest chain wins actually means that the chain with the most cummulated proof or work wins. These two roles in a Bitcoin or Ethereum protocol combined into one, however they not necessarily must be always combined. Cummulated proof of work could be actually separated from the consitency of the blockchain, as there might be requirements where the cryptoeconomical security of a given transaction should not be increased without end but might be limited after a high enough number.
