Too Long; Didn't Read
Testing allows us to check that every behavior is exactly how we have coded it. The beautiful thing about testing is that you are able to follow the exact point in which your function has problems. It helps your team understand more about the x or y function since you will have a descriptive name that gives the reader what you are testing. There are many types of testing. The one that I've described is a unit test. It focuses only on a single unit, and it doesn't care about the connection of your function with other parts.