Pretty sure you use it because vercel likes the frontend engineers who don't know how/or care to deal with the backend.
I want to leverage Next.js but not Vercel (if I don't have to). I have experience with backend and devops, so I care about and want to deal with the backend myself (cost-savings, control, decoupling).
The sentiment and complaints seem to sum up to, "if you're using Next.js, you basically [or eventually] have to use Vercel." But I do not want this to be true. That said, I also don't want to sacrifice benefits of Next.js or have to "fix" things constantly or unnecessarily.
If it runs in the browser, it's frontend. Server? Backend.
There's SSR but the name itself clarifies: Server Side Rendering.