The County has municipal responsibilities In the unincorporated areas---collectively, the UnCity. Unincorporated areas do not have Mayors and City Councils to attend to local priorities. So they must rely on the Board of Supervisors to care for their communities.
According to the County (http://www.municipalservices.saccounty.net/Pages/default.aspx.) the municipal services for which it is responsible for include: garbage collection, animal care, road maintenance, code enforcement, clean water, storm protection, building permits and development services, open space conservation and regional parks. In incorporated areas, cities typically provide those services.
Sacramento County is an oddity among California's counties due to the very large percentage of residents and high degree of urbanization in its unincorporated areas. For example, with 100,000 or so people, Arden Arcade would be among California's largest cities if it was an incorporated municipality and, at about 5500 people per square mile, is considered "highly urbanized" by the state. But our community is just a small blip on the local radar - Arden Arcade has less than 1/3 of our County Supervisor's 315,000 constituents, which means our community is easily ignored.