First, as stated before, job descriptions are not an ISO 9001 requirement. It is, however, a common way for organizations to meet the requirements of 6.2. For the organization to define the necessary competence on the basis of appropriate education, training, skills, and experience, it's quite natural to meet this requirement by establishing job descriptions and then ensuring the people who fill those positions meet the requirements.
If you're planning on using job descriptions to help meet this requirement, be sure not to document yourself into a corner. When you set a requirement, you have to meet it. The difference between the word "required" and the word "preferred" is huge when you have to explain to an auditor why someone doesn't exactly meet the job description that was established.