Because it adds flexibility. By decoupling the frontend from the backend it's much easier for other devs not directly affiliated with the server repo (e.g. Ollama) to design new frontends that can connect to it.

I also think it allows experts to focus on what they are good at. Some people have a really keen eye for aesthetics and can design amazing front and experiences, and some people are the exact opposite and prefer to work on the backend.

Additionally, since it runs as a server, I can place it on a powerful headless machine that I have and can access that easily from significantly less powerful devices such as my phone and laptop.