Ensuring that your app functions as it should is absolutely critical. When your app is small, it's tempting to leave the testing as an afterthought. After all, it's a small app and it is reasonably easy to test by hand, right? Unfortunately, even a small app that targets more than one device is immensely complicated to test, simply because you wouldn't have all the devices available for testing. Furthermore, it's incredibly boring to test your app by hand after each change on every platform; you will make mistakes doing so. Thankfully, there's a better way.
It's unfortunate that testing often gets left as an afterthought, because it's an incredibly important part of the development process. Unfortunately, it's easy to overlook it when you're first learning a language, since you're usually assumed to be working with small, easily verifiable portions of code and not a large multi-thousand-line codebase. Why write a test when a simple twenty-line program...