A Trade Route provides yields each turn to its origin city, primarily based on which districts are at its destination city. For example, trading with a city with a Holy Site will provide Faith, and trading with a city with a Theater Square district will provide Culture. Additionally, Religion and [Icon_Tourism] Tourism can spread along [Icon_TradeRoute] Trade Routes.
It also gets yields based on the path it takes. If it passes through a Trading Post you own in a foreign city, it is worth an additional 1 Gold. Long paths which pass through several Trading Posts are very valuable.