A job description sets out what a role is responsible for, what it requires, and how it is judged.
What it is used for
Hiring. It is what the advertisement is written from, and what candidates are assessed against — see recruiting.
Performance. A shortfall can only be discussed against a stated expectation — see performance management.
Classification. Whether a role is entitled to overtime turns on the duties actually performed, so the written duties matter — see wage and hour basics.
What it should contain
The purpose of the role in a sentence.
The duties, in order of how much time they take rather than how impressive they sound.
What is required: licences, certifications, experience, physical requirements, driving.
Who it reports to.
What good performance looks like, stated concretely enough to be measurable.
Where they go wrong
Written from the last person who held it rather than from what the business needs now.
Aspirational lists covering everything the role might touch, which makes the actual priorities invisible.
Never updated. A description three years out of date is worse than none in a dispute, because it documents an expectation nobody was working to.
Small businesses and "everything else"
Real roles in small businesses are broad, and a catch-all line is honest. It should not do the work of the rest of the document, though — if most of the job lives in the catch-all, the description does not describe the job.
Keeping them current
Reviewed when someone leaves, since that is the moment the business is deciding what it actually needs, and the cheapest time to change the shape of a role.