Testing will give you the opportunity to find and fix bugs before the software is deployed in the field. How do you know if code works if you don't test it? Sure, it may compile, but does it work? You don't want to be the developer that causes a billion dollar error on a space mission to Mars, a regional blackout, or worse, death, just because you didn't run a simple test, do you?
There are numerous kinds of testing you can do right from your IDE. Testing is important to verify that the software works:
- as expected according to the requirements
- in the deployment environment
- within a reasonable amount of time
- without failure when pushed to it's maximum
- without failure when given incorrect input
- ...
Further, you need to log when you test. And you need to re-test on change. How can you be sure the change you made doesn't affect other portions of the system if you do not test them continuously?