Progressive enhancement
Progressive enhancement is building a page so that its core function works without JavaScript, and then improving the experience for clients able to run it, rather than depending on scripts for the page to work at all.
For a storefront the core function is stating facts and expressing actions: a price in the markup, an add to cart that is a real form posting to a real endpoint. The script then intercepts and does whatever it does now.
It is not a request to abandon a modern front end. It is a floor rather than a ceiling, and for a shopper with scripts enabled nothing changes at all.
Why it matters to a store
It is the single technique that resolves most agent readiness failures at once, because almost every one of them is a place where the browser was doing work an agent will not do.