The United States Constitution preserves the notion that the country is a federation of semi-sovereign states and that powers not specifically granted to the federal government are retained by the states. States, therefore, are not merely provinces or subdivisions of federal administration.
State governments in the U.S. are relatively powerful; each state has its own independent criminal and civil law codes, and each state manages its internal government.
The governor thus heads the executive branch in each state or territory and, depending on the individual jurisdiction, may have considerable control over government budgeting, the power of appointment of many officials (including many judges), and a considerable role in legislation.
The governor may also have additional roles, such as that of commander-in-chief of the state's National Guard (when not federalized), and in many states and territories the governor has partial or absolute power to commute or pardon a criminal sentence.