Back in the late 1970's with airline deregulation, air carriers had much freedom to choose which domestic routes they wanted to fly. Favoring larger markets which offered high passenger loads and yielding higher returns often meant bypassing services to smaller communities.
So the government stepped in and introduced the Essential Air Service Program (EAS). Rural cities distanced more than 70 miles from a medium or large commercial airport qualified for a partial federal subsidy. Greater than 210 miles - no cap on the governments subsidy. So while United Airlines/Skywest Airlines may not want to provide air service from San Francisco to Crescent City, on the far north coast, a nice kickback from the government helps them reconsider.
Approximately 140 cities across the United States are part of the EAS program, mostly throughout the Midwest and Alaska. In California, SkyWest Airlines to Crescent City, and Air Midwest, serving Visalia and Merced, receive federal funds of up to $1 Million each year.
Opponents of the program say having service to rural communities promotes business opportunities, and better connects people to the rest of the country.
Critics say the essential air program is actually unessential, citing many flights actually depart with only a couple passengers, an unnecessary government expenditure. They also argue that since the program was developed nearly 30 years ago, regional airports have absorbed much of the demand coming from rural areas, making this program unnecessary.