Because I have just spent 20 minutes trying to figure out what 1, 3, 5, and 6 mean in regard to access levels. Had the previous developer added a single comment to the database design to say 1=guest, 3=admin, 5=customer service, 6=suspended or the like my job would be more manageable.
Developers should document at a minimum within the code and in an external document. Further documentation can be injected into many other places such as the database table design via Enterprise Manager. Once you make commenting a habit, the pain goes away and it almost becomes fun.