Old code without comments is the worst-case scenario, of course. Comments give us some hope because they explain different segments of the code. But there can be major differences in commenting style. The developer who is reusing the code months later will probably assume that the original developer commented the code properly. This may be a big assumption.
Developers nowadays come from all across the country and all over the world.
Differences in training, native language and writing skills may cause two developers to describe a line of code using two completely different phrases. Under these circumstances, comments may actually confuse more than they clarify. They waste the time of the second developer, and they offer no guarantee of proper interpretation. The second major benefit of Design by Contract is related to the state of the job market. With such frequent turnover in our current software development workforce, the importance of providing clear information about our code is at an all-time high.