Unit and functional testing is essential for the long-term success of a project. There are a lot of reasons why writing and running tests will greatly increase code quality. Here’s yet a few more. Four to be precise. 1. Tests Make for Purposeful Code Generally speaking, a single test should only have one assertion. Each function or method should only have one test. Naturally, then, each method or function needs to be written to do one thing and one thing only.