I use a similar system when building mobile app features.

Sometimes (almost always) an app feature is developed parallel to the API service.

It's useful to return a mock response so you can build the entire feature including networking. This way you don't need to wait for the server side component to finish.

A temp server takes time to set up and maintain. It's like any tool. Why use Dropbox if you can just save your file to a usb drive and carry it around everywhere?