Every software project that's deployed suffers from bugs that are inevitable--and bugs are time and money consuming.
Using a Test-Driven Development (TDD) approach, where you write tests alongside the code you are creating, will not always improve the quality of your project, but it will make your team more agile. This means that the developers who need to fix a bug, or refactor a part of an application, will be able to do a faster and better job when relying on a battery of tests. If they break a feature, the tests should warn them about it.
Writing tests is time-consuming at first, but in the long run, it's often the best approach to make a project grow. Of course, it's always possible to write bad tests and end up with poor results, or create a test suite that's horrible to maintain and takes too long...
