Apps that are in Alpha and Beta Modes or Phases are undergoing testing.
Apps and software go through four phases of development before they are released to the public as the "finished" or versions of the app or software. These phases begins with the Pre-Alpha Phase, followed by the Alpha Phase, then the Beta Phase, and finally the Release Candidate Phase. Technically, an app goes through testing in all four phases. However, it is during the Alpha and Beta Phases that formal testing takes place. Thus, it is during the Alpha and Beta phases that an app undergoes intense feature developments and enhancements. It is also during this phase that major bugs and crashes are fixed.
This means that when an app is still in the Beta and Alpha Phases, it could still undergo major changes such that the final release could be very different from its Alpha and Beta versions. How different will depend on the people testing them, particularly the engineers and developers who conduct the testing during the Alpha Phase and the actual users who conduct the testing during the Beta Phase. So, those who are using Alpha and Beta versions of an app should not be surprised if during the app's final release, it is suddenly missing a few functions or has new features that weren't there during Beta.
Another implication is that apps in Alpha and Beta Phases could still be buggy and unrefined. They are still being tested, after all, so most bug fixes won't occur until after the Beta Phase. Thus, users should not have high expectations about an app while it is still in Beta and, much more so, during the Alpha Phase.
Finally, if an app is still in Alpha or Beta, then this only means that those using it are actually serving as "testers", providing invaluable information to contribute to its development. Thus users should be taking note of the issues that they encounter, so they can give valuable feedback to the app's developers.
Hope this helps.