During the development and testing phases of a mobile app project, you may need to install the app on the internal testing devices, or even the client's phone for review and feedback. Also, most apps will have a web api that needs to be hosted on a server.
During the development process, the app code and the Web API code often need to be updated together, so they match. For example if you change the ROUTE of a Web API method, or its signature (parameters, etc) if the appropriate changes are not applied on the app as well, it can break.
Consider the following scenario:
Then after that:
Wait a second!
On the testers' or the client's device, the app is still v1 and it's pointing to the same web api URL, which is now hosting v2. As a result, the app can break and you will have to physically gain access to the device again to install v2, which would be very annoying due to the wasted time, confusion and possibly travelling costs.
To address this problem you need to ensure that whenever you install an app for a tester or the client, the web api will not change after that, for as long as you need that version of the installed app to work.