The Market is a Tier 1 Amenities & Services Building that generates Income in the form of Gold Ingots and provides Desirability for the player Settlement.
Markets are critical to the availability of food and other materials to Shelters, with the grocer stocking resources in villager Shelters for consumption and reducing the travel time required for villagers to meet resource needs.
Markets begin with a limited set of resources that will be stocked and employ a maximum of 1 Grocer, upgrading to a Market Square increases hirable maximum to 2 Grocers and expands the categories of luxury resources that marketplaces will stock.
Income
Markets generate 1 
(2 after upgrade) on a monthly basis from the following Buildings within its radius:
Market income is distinct from monthly income paid by Shelters at tier 2 (homestead) and above, as well as monthly income from consumed Luxury resources and beer consumed at Pubs.
Additionally, upon upgrade, the radius of gold collection increases slightly, as does the desirability generated for shelters by the presence of this building.
Basic Strategy
When placing a Market, it has a workable radius (13 squares from center - 11 tiles from outer market edge) as an area of effect. Each Shelter within the working radius increases the monthly Income accordingly, but overlapping work areas from multiple markets will not generate more income for the same Shelter.
While not strictly necessary for Shelter function and development, Markets should be placed in close proximity to as many Shelters as possible while leaving room for other Desirability generating Buildings, such as a School, Pub, or Bakery.