While an MVP could be something as minimal as a landing page, announcing your app, or a live mock up version of your app, there comes a time that your app should be a little more than that, whether it is to prove your next hypotheses or to see the actual thing in its most basic shape in action. It's about time to create a Proof of Concept (PoC).
Standalone apps are rare these days. Most apps have functionality to share content on Twitter or Facebook, have leaderboards (if it is a game), let the user post pictures or video, have a chat or otherwise communicate with each other, and so on. For this your app needs to have a backend.
You can of course create your own API or use the API of the many solutions that do exist for this purpose, the so-called Mobile backend as a Service (MBaaS). These solutions do work like any other Software as a...