Imagine you're a new node, and you've just connected to the network to download the blockchain. You get data for multiple different blockchains, which one is valid? The genuine blockchain is the oldest, but it's not obvious. We have to add time to the blockchain to prove it's age. Work is energy over time, therefore, POW is proof of time.
The only reason it's applied to a block hash is to create a delay, the block interval, and to enable the network to regulate it. It does not secure block data any better than a natural hash. POW can be applied to anything in a block and it would work just as well.