ASP.NET MVC with React - how to use them in a combination

I have a newbie question. If we use React (or any other similar framework) in ASP.NET MVC we end up with very simple (single?) cshtml file which delegates UI rendering to React. So in that case, why bother with MVC? Shouldn't be a better idea to create WebAPI REST service, and a frontend client application in pure JavaScript/React as two completely separated applications?

I just don't get the idea of using ASP.NET MVC in combination with different frontend frameworks...