Because it is the law. Massachusetts Building Code (780CMR 5110.1.0) states: "It shall be unlawful to construct, reconstruct, alter, repair, remove or demolish a building or structure... without first filing a written application with the building official and obtaining the required permit therefor." In addition, a building permit is for your own protection. The building inspector will require the work performed to meet the standards of the Mass Building Code, which translates into your security. Insurance companies may require a Certificate of Occupancy or approval of the building official for certain work. If you are a licensed contractor, failure to secure a building permit could result in the loss of your license.