In a Proof-of-Stake System, What Is the Equivalent of a “Miner” and How Do They Benefit from Front-Running?

In a Proof-of-Stake (PoS) system, the equivalent of a miner is the "validator." Validators are responsible for proposing and attesting to new blocks. They benefit from front-running in the same way miners do: by observing profitable transactions in the mempool (or private relays) and ordering their own transactions before the victim's, capturing the arbitrage profit.

They are incentivized by the higher priority fees and the extracted MEV.