The short answer is so that potential volunteers can easily see what is expected of them.
A good job description will tell them exactly what their responsibilities would be, how much time is required for the volunteer position, who they can talk to if they have questions or concerns, what training will be provided for them, and the length of time they are agreeing to volunteer for.
A good job description is a wonderful tool in the hands of a recruiter.
It's something tangible to give to a potential volunteer. The recruiter can suggest they take it, read it over, and pray about the volunteer role before they give an answer.
Here's a template you can work from. It includes all the important pieces of a good job description. You just fill it in, customizing it to your specific role. You can also use this template as a jumping off point to create your own.
Here is a guide for using the template. This guide describes how to fill in the job description template along with some hints and tips.
Let me encourage you to create job descriptions for children's ministry positions in your church! They are very helpful when you are recruiting. They are also great to have when a current volunteer has a question about their volunteer position.