The term ‘job description’ is a flexible one, but here I’ll be using it in a modern context, to describe all of the copy you’d use on an online job board. It usually includes information like job title, duties, salary, etc.
The purpose of a job description is to persuade, as well as to qualify and disqualify candidates before you spend time interviewing them. If you’re looking for someone with a degree in economics, then putting that on the job requirements list will disqualify the candidates without the necessary qualification. On the other hand, the right copy and tone can help you stand out to incredibly desirable candidates.