Every state has its own rules and regulations for what requirements a builder must meet in order to perform building or remodeling work. These rules are in place to protect homeowners from fly-by-night or fraudulent individuals who claim to do remodeling or building work. There are structural, safety, and systems information that builders are required to know (or know how to research effectively). Most licensing requirements demonstrate the builder has a basic knowledge of construction materials and methods, competency to navigate the code manuals, contract law, and conflict resolution.
Every state is likely to be a little different from the next. Some states require that any project with a total value exceeding a particular amount requires the person responsible to also be a licensed contractor. Some municipalities require that work over a certain amount must be permitted and inspected by that municipality. Some states allow for licensing in a particular trade. Some municipalities require licensing for specific trade work.
Before starting any project, or any building/remodeling business, it is always a good idea to research your own state and local requirements.