The Harbor increases that city's income from trade routes. It increases a city's health from access to Clam, Crab, and Fish resources.
Just because a city is built in a coastal region doesn't guarantee that the city is readily accessible by ship. In order for a port city to establish a steady trade, fishing, or other shipping industry, the city must have a harbor. A harbor is a protected body of water that opens into an ocean or lake that shelters ships from waves and high winds. Although some coastal cities are established in areas where a natural harbor exists, most seaports are forced either to improve the existing natural harbor, or to build a man-made harbor to shelter ships and provide channels deep enough to accommodate large vessels.