You need some form of backend if you want Server Side Rendering, and SSR is why most people opt-in to a React framework. And then you already have a backend so

You need some form of backend if you want Server Side Rendering, and SSR is why most people opt-in to a React framework. And then you already have a backend so it's an easy jump to just using Nodejs on your backend as well. But a lot of people also just use NextJS as is and never build any server side code and just run their own API beside it. Or use other frameworks that don't include any SSR features, such as Create-React-App