Job descriptions tell you what the knowledge, the skills and the abilities are, which enables you to go out and look at the market. This enables you to compare like jobs with like jobs when you want to determine what you’re going to pay for a particular job.
Without a job description, you’re not really comparing apples to apples. If you’re just looking at job titles you do not know the actual job duties of the positions. For example, an administrative assistant can be anything from a basic office clerk duties who opens the mail and handles correspondence to making major decisions for an executive while they’re not in the office. Both jobs have the title administrative assistant but it does not necessarily mean that they would make the same pay.
Job descriptions also help determine the worth of each job to an organization. You might have a job in your organization that is unique to your company, but if that job is worth the same as another job in your company (called a Benchmark Job), you can easily determine the rate of pay for that position in the market using the benchmark job.
Also, if you’re recruiting for a position and you don’t have a job description you won’t even know how to write the employment ad nor determine who you’re looking for, what you require of that position or how to “sell” the job to the candidates. The job description will provide you with the necessary information to successfully recruit.
If you have more questions, I suggest consulting with an experienced HR professional.