Job descriptions are usually essential for managing people in organizations. Job descriptions are required for recruitment so that you and the applicants can understand the job role. Job descriptions are necessary for most people in work. A job description defines a person's role and accountability. Without a job description it is usually very difficult for a person to properly commit to, or be held accountable for, a role. This is especially so in large organizations.
As an employee you may have or be given the opportunity to take responsibility for your job description. This is good. It allows you to clarify expectations with your employer and your boss.