Why do I need to reinstall ME to get rid of a problem?

You rarely need to reinstall AME from scratch. But we always recommend to keep your applications current if you don't auto-update. Also cleaning up preferences, media cache or your queue may help if you run into issues. I agree that a better guidance would be very helpful. 

Doesn't Creative cloud auto update apps anyway?

By @avpromedia

Only if you choose that in your settings.

This is resolved by uninstalling ME, then installing it again.

But why is this necessary?

By @avpromedia

Sometimes, for various reasons, installed programs become corrupted.