I'll go even further: Extensive unit testing improves your customer experience and the design in your code because it forces you to decouple code that's bound too tightly together.

The topology editor creates a matching bound virtual image unit (Figure 20) that contains disk definition units and network connection units, according to the data in the virtual image file.

This process is of Paramount importance, because future maintainers of your code are bound to tamper with the original logic at some point, and the unit tests will fail the moment something breaks.