A very simple answer! When you're investing your dollars in a home, home improvement or commercial venture, these reasons assure you of quality workmanship and a warranty that the services provided will meet the various local, state and federal building codes. You have recourse if they do not!
Unlicensed contractors may not meet the standards required for performance or compliance to the various building codes that may be applicable. Poor workmanship and non-compliance can cost you dollars! Are you willing to risk your money? Using an unlicensed contractor may be equated to spinning the roulette wheel!