Regulations cap downside. Benchmarks set the terms of how to measure upside and inspire competition by making it measurable.
Benchmarks are an emergent schelling point.
Someone sets rules and a way to measure quality.
No one has to use their rules if they don't find them valuable, but if people do, then other people will also want to show they can do well on it, which is a compounding loop.
People take it seriously because other people take it seriously, and people take it seriously because every marginal person who considers taking it seriously looks at it and agrees that it sounds plausibly useful enough to take seriously.
Benchmarks can get a compounding amount of momentum in proportion to their quality.