To the user? Absolutely none. You can make better sites without it.
To the publisher? Originally, some twisted incentive that you'd rank higher on mobile and then globally but IIRC this no longer exists and OG data is also used. AMP is a component library too now so I guess that could simplify development if they couldn't use anything else, because HTML is so hard.
To Google? A funnel away from in-house and management at larger publishers, and ideally into one of their AMP-supported models. AML is "open" now but it's still Google, innit.
Google can load an amp page instantly from a search result page since they can cache the data and pre-render before you click. No publisher can do that with their non-amp pages.
