What does PoW achieve that PoS doesn't ?

They both tend to be centralized:

In PoW, people with the most money buy most mining equipment and mine more, creating the same kind of mining centralization PoS could create.

I'm not sure how PoS creates a notion of 'decentralized clock' PoW can, I'm not sure how PoS dictates how fast are blocks validated.. is that the key ??