Never also, don't understand at all. Bugs usually visual and business process. If wanted to type all business process to unit test not sure client willingness to paid the time.
Unit tests are no silver bullet. This trend was started by consultancies that found more ways to milk customers - write unit tests and do inline code (!) documentation. Both are a real cash cow when the client doesn't know what they want and you need to constantly tweak code and respectively tests and documentation! I understand API documentation, but internal code that's obvious like what "width" and "height" parameters of a method are for is beyond stupid!