Having job descriptions for staff is essential aspect of an effective human resource department. Aside from letting potential hires understand what the job entails, the job description is essential for a variety of other reasons as well. It sets expectations regarding what tasks the role is responsible for. It ensures staff are aware of their expected performance metrics. It helps onboard covering team members, etc.
The job description ensures that you are paying for essential, performing staff. All staff members are expected to accomplish particular tasks and provide benefit to your organization by ensuring those activities are actually conducted. If you clearly specify your expectations to your team in a written document and the employees sign off on those tasks, it makes it that much more aware to them of what is expected at your organization for their particular role.
As your employee signed off on the job description, they are also agreeing and accepting their understanding of those duties, and agreeing that they are competent to perform them. This ensures that all job requirements are understood, and allows you to therefore rate their performance on those aspects during their employee reviews etc.
If an employee is injured, takes time off work, etc. you can also provide their job descriptions to covering or new staff replacing their duties. This allows for a smooth transition, and minimizes time of delay of covering duties that staff didn't know was included. It also allows the new or covering staff to discuss any questions they may have about particular duties that they will be taking over.
Lastly in the case that a staff member is out for FMLA etc., or needs a work modification, it is clearer about what is involved in the role and where modifications can be established. As well as what type of role that the employee will return to upon their recovery.