HYDROGEN is made to create fully customizable shopify based E-Commerce web apps. Since its based on Nex.JS we have features of nextjs available to us and it also comes with many predefined components and hooks to ease your development.
It also comes with built-in cache defaults support for non-blocking data-fetching, ensuring a fast shopping experience.
And it uses Vite for bundling instead of Webpack
Hydrogen also uses shopify's GraphQL api to steam data easy and efficiently.
Shopify is also going to launch a hosting service for Hydrogen APPs called Oxygen.
Hydrogen is react(next.js) based framework, so if you already know react and you develop custom storefronts it might be a great idea to leverage the extra powers that Hydrogen gives you on top of Next.JS