The United States operates on a federal system. This means that state-level governments exist between the national and municipal level. The individual states enjoy a certain degree of autonomy. They can pass laws independently and exercise a significant amount of local and regional control. Technically, state laws are subservient to laws made on the national level, but it is up to the federal government to choose to enforce this. The 10th Amendment to the United States Constitution allows states to make any law that concerns a matter not covered by the national government and constitution.
A unitary government, by contrast, does not have mid-level governments such as provinces or states. Nearly all power is concentrated in the central government. Municipalities may have some local control, but their laws and governments are greatly subservient to the central government. Also, since laws are passed on a country-wide basis, sometimes they overlook or do not address regional issues.
As a federal system, the United States generally allows for more civic engagement by citizens. It is easier to participate at a state level to influence local and regional politics and policy.
On the other hand, unitary governments trade the broader participation of citizens for greater efficiency. These governments do not have the need to pass legislation through the increased levels of bureaucracy that federal systems have. As a result, changes can be made more quickly, but usually with less transparency and citizen involvement.