Everyone agrees that testing is a good thing. Not everyone agrees on how much testing is cost-effective and what exactly the kind of testing is right for a specific piece of software or product. But automated unit tests cam help you ensure your software is healthy.
Unit tests operate on the smallest possible section of code, on logical modules. In software design, a program of any decent size will be broken down into modules. These modules have inputs, outputs and behaviors. These inputs and outputs and behaviors are used by other parts of the program to perform work and can be monitored or tested by the test software.