In addition to regular unit tests, there are numerous other types of tests that can be useful in their own way, such as automated UI tests. You could, of course, write a lot of test scripts and go through them each time you release a version. If your app is small and you are the only developer, that could work fine. But as your app starts to grow, you are supporting more and more platforms, and as your development team grows, doing these kinds of tests becomes undoable by hand.
Luckily, this is something that can be automated. Xamarin does ...
