Web API talking to Vue app hosted separately in IIS... why is app.use spa needed?

Question

I have an ASP.NET Core 6 Web API (converted from 2.2), I cobbled up from tutorials.

In the startup section I have this:

app.UseSpa( spa =>
    {
        spa.Options.SourcePath = ".";
        spa.UseVueCli(npmScript: "serve");
    });

Which appears to be for hosting a Vue app inside my Web API. My Vue app is hosted separately in IIS. But this code is still required for the Web API to successfully communicate to the Vue app.

What exactly is it doing to allow that?

It really seems like this should be unneeded, as I don't have a Vue app in the Web API, so why am I starting a node process?

I'm so confused. Links to relevant documentation greatly appreciated.