The whole idea of next.js is that it‘s the solution for backends and frontends. Using it for your frontend only makes absolutely no sense at all. Or is there something I‘m not seeing here?

You can use it as a standalone backend or you can just use it as a router / renderer "web backend" but then still have an API off somewhere else. That API could be a monolith, a bunch of microservices, power multiple front ends, be a legacy system etc etc.

When I said backend above I meant company backend / API / services.

Ignoring the API routes in Next.js is perfectly valid, the build system and built-in routing and utilities make it a better frontend-only framework than anything else in the space by far (CRA, etc.)

Using it with a separate backend/API is also sometimes necessary, difficult to use serverless functions if you want a WebSocket server or similiar.