8 Benefits of Unit Testing

1. Programming with Confidence: unit tests increase your confidence about the code you are developing because the tests will clearly show if the changes you made caused the failure of some part of your code. This way you can fix the code, run the tests again, and if all pass, you can continue to work with the confidence that everything is still working properly.
2. Continuous Integration of Code: unit testing ensures that the new features developed didn’t cause side effects on the code that was already working, so you have more confidence that the new features developed are working properly and also that they did not harm other parts of the software that previously had been already checked and perhaps even validated.
3. Better confidence on the quality of your code: with unit testing, you are not only supposing that your code is working, but instead you are sure about it, because when all your tests are passing you know that your system is behaving exactly the way you expect it to behave.
4. Elimination of Bugs When They Arise: with unit tests you discover bugs in your code as soon as they appear. This way you can eliminate them from your system immediately when they arise, avoiding side effects that could make these bugs become much more difficult to locate and much more expensive to be corrected when more functions and more bugs would have already been inserted into the system.
5. Less Time Spent Debugging Code: with unit tests, you do not need to spend lots of time with debugging looking for the origin of bugs because the tests will indicate what is failing or not in your system. Thus the programmer has greater productivity because he/she will spend more time with programming than with debugging of code.
6. Fast location of bugs: unit tests help you to know exactly where the bugs are in the code, what you should correct and where this fix should be done .
7. The functional testing phase is less overloaded: when the system is designed with unit testing, the testing team will find fewer errors that make the program stop working unexpectedly or bugs that cause the system to behave wrongly. Thus, the testing team will have more time to identify gaps related to business rules of your software than with reporting of errors like NullReferenceExceptions or StackOverflows, things that often prevent or hinder the testing team to perform complete testing of your system functionality.
8. Better reputation for the programmer: one of the things that harm much of the reputation of a programmer are the frequent reports of failures and errors found in features developed or maintained by this programmer. With unit tests you will deliver code with greater quality and hence you will have a better reputation due to a smaller number of problems found in the features that you develop.

Source- http://csprogrammer.net/en/nunit-tutorial-part-i-introduction-to-unit-testing/