If you are a small, small business (under 5 employees), you are unlikely to have an employee manual. I’m a big proponent of them so long as they are actually followed. The biggest drawback of these written manuals is that they are written and a lot harder to change then just general policies in a small office. As an employer, you are obligated to abide by the manual. Failure to do so, may result in liability.
The biggest upside, which negates all downsides, is that you have established clear expectations of your employees and a guidelines for you as the employer to ensure you are within the law. Assuming the manual was created by your business attorney, the manual is often a restatement of already established legal policies. Another reason to keep it in hand at a reference reach.