You want to work with a contractor that has a license to do the work he claims he knows how to do. You also want him to be insured in case one of his employees is hurt in your home so you are not held liable.
This is important because if something should go wrong in the home improvement process and should the contractor be at fault, you can sue them for damages. YOu can also look up their license in the state database.
These steps ensure your contractor is working legally, is technically capable, financially responsible, and reliable. You have potential exposure to a mechanics' lien and a mistake could be expensive.