The jargon-free answer to the question is that if you don't have a good idea of what an employee is supposed to be doing, it's hard to evaluate how well the employee is doing it. Most job descriptions are focused more towards hiring than evaluation. To develop metrics that can be used to evaluate performance, one needs to perform the sort of job analysis that details not only the business priorities of the various tasks assigned to an employee, but also the purposes of each task, and the ways the task contributes to the business. For example, if an employee's job includes answering phones and directing calls to the appropriate division, a simple description would enable one to evaluate merely whether the phones were answered. A more complete analysis would note that this employee is the first person a customer encounters, and thus is crucial to establishing corporate image and customer relationships. This analysis means that rather than simply evaluating whether phones were answered, a supervisor could distinguish employee excellence (cheerful friendly tone, answering in detail questions from customers who don't know the right person to contact, helpfulness) from mediocrity.