Why does order between UseStaticFiles and UseDefaultFiles matter?

From the docs:

UseDefaultFiles must be called before UseStaticFiles to serve the default file. UseDefaultFiles is a URL rewriter that doesn't actually serve the file. Enable Static File Middleware via UseStaticFiles to serve the file.

The order of middleware does matter, this is why, for example, UseStaticFiles has to come before UseMvc as the MVC engine will handle all of the requests. In this case, UseDefaultFiles is simply rewriting the URL and passing it on to the UseStaticFiles middleware to serve.

Taken from the documentation on Static files in ASP.NET Core (under Serve a default document, there's an important note).

UseDefaultFiles must be called before UseStaticFiles to serve the default file. UseDefaultFiles is a URL rewriter that doesn't actually serve the file. Enable Static File Middleware via UseStaticFiles to serve the file.

Based on this, it's important to first setup the URL rewriter (UseDefaultFiles) before serving the actual file (UseStaticFiles).
If you don't, the UseStaticFiles middleware will kick in first, but a request to the root of the application won't tell the middleware which 'file' to serve. When you make sure the rewrite is in place first, a request to the root of the application will have been rewritten to be a request for (one of the) default file(s).