A handbook sets out the rules and expectations of employment in one document given to everyone.
Why it helps
It answers the recurring questions once rather than differently each time somebody asks.
It establishes that rules were communicated, which matters when one is enforced — see terminations.
And it produces consistency, which is the single strongest defence to a claim that someone was treated differently — see employment practices liability.
What it typically covers
Hours, pay periods and overtime.
Leave: vacation, sick, and any required by law locally.
Conduct expectations and the process for addressing shortfalls.
Safety requirements, and vehicle and equipment use.
Harassment and discrimination policy, including how a complaint is raised and to whom, which several jurisdictions require.
Technology and confidentiality.
Where it creates risk
Language that reads as a promise. Describing a progressive discipline process in mandatory terms can bind the business to following every step before a dismissal, even where the conduct did not warrant the delay.
Policies that are not followed. An unenforced rule is worse than no rule, because the departure from it is the fact in dispute.
Boilerplate not adapted to the jurisdiction. Leave and pay requirements vary by state and by locality, and a generic handbook is frequently non-compliant.
Acknowledgement
Signed on receipt and retained, because the value depends on being able to show it was given — see employee onboarding.
Review
Employment law changes. A handbook written once and left is progressively less accurate, and an annual review by someone who knows the jurisdiction is the usual approach.