In addition to the information in Andrew Timpson's answer, it is important to understand that the information is critical for the employer itself. A company with even one employee in another state is frequently considered by the other state to conduct business in that state. The employee's location may be considered a branch office of the employer; employer provided property (such a laptop) is considered employer property in that state and may be subject to sales/use tax. As a foreign (i.e., out of state) employer doing business in that state, the employer will have to obtain from that state a qualification to do business there, file tax returns, pay various state mandated benefits, etc.
And, in some places (such as New York City), the whole approach is again applied at the local level.
Frequently, it's a major PITA to comply with those requirements, which is a reason employers try to avoid getting into that situation to begin with. That, incidentally, may be one of many problems with allowing employees to work from home on a regular basis.