Significant value which is given gratis by said peers, which journals use to boost their reputability and, by association, their profits. Publishers are profiting off of free labor from subject matter experts. Even more disappointing is this free labor is viewed as a right of passage. Don't forget that the author spent hundreds to thousands of dollars to access these unpaid peers. Publishers are increasingly well-known as scammy. It's why MIT ended their Elsevier contract, and why many other R1s are following suit.
Also don't get me started on the dubious quality of peer review in todays "Publish or Perish" climate.