Safety is the primary reason for requiring permits.
City inspectors check to make sure construction and installations are proper for the protection of your family and property, you neighbors and buyers of your property. If you're having a contractor do your work, the contractor can obtain the necessary permits, but you should check to make sure the contractor has done so. The cost varies depending on the type or project, but it is not prohibitive. These units will have been tested under extreme conditions and are approved for installation with specifications for distances these units can be placed from combustible materials. There are appliances that are not approved or tested for safety.