You should understand that early bug detection saves a huge amount of project resources and reduces software maintenance costs. This is the best known reason to write software tests for your development project. Increased productivity will soon be evident.
Additionally, writing the tests will give you a deeper understanding of the requirements and the problem to be solved. You will not be able to write tests for a piece of software you don't understand.
This is also the reason behind the approach of writing tests to clearly understand legacy or third party code and having the ability to confidently change or update it.
The more the code covered by your tests, the higher would be your expectations of discovering the ...