Just like an employee handbook, job descriptions can be used evaluate an employee. Are they living up to the general duties required of them. It can also be used as a defense when making a decision to terminate an employee for poor performance.
Without a job description, the employer doesn’t have a defense against any type of discrimination. Job descriptions work like employee handbooks in that there is written proof to support the issue at hand.
Job Descriptions are a documented way of showing what the expectations are both from the employer perspective and the employee perspective. Should there be any type of question about a position, there’s something in writing.
Keep in mind, job descriptions should be concise and not include every detail of the job and do not include procedures but only contain the basic duties of the job.
If you have more questions, I suggest consulting with an experienced HR professional.