The vast majority of states require employers to carry workers' compensation insurance, which helps pay lost wages, medical expenses, and rehabilitation costs for employees who are injured while doing a work-related activity.
Once you have at least one person working for your business, you're an employer. Being an employer comes with a wide range of responsibilities, including withholding, depositing, and paying employment taxes.
The IRS says, "All taxpayers should review their federal withholding each year to make sure they're not having too little or too much tax withheld." However, employees may not be aware of the IRS' suggestion, which is why employers should tell them about it.