Your company is developing an Android app. Along the way you want to distribute test builds of the app. The designer needs to do user testing out in the field, the backend developer wants to end-to-end-test the whole system from the app… oh and by the way: the CEO wants to check in periodically as well.
If you were building a web-app it would be easy: just email a link to everyone. Distribution happens every time they reload their browser. However, with native apps you need to periodically push out new versions of the app to their phones.