Trade is vital to building infrastructure. If you have a marketplace and you want to build a bank, you must decide if these shields are worth the effect. The upkeep is 3. The profit from the bank and MP must be higher than the upkeep for the bank to be profitable.
You irrigate squares to grow your cities. You road squares to generate trade, and yougenerate trade to be rich. Roads give you trade, population growth by irrigating squares will give workers to exploit the roaded squares, and MK, banks, and Stock exchages exploit the trade by making gold from the trade. If there is not enough trade from a city, there's no reason to divert setters to grow pop, to build improvements. The city will remain backwards and primitive.
Trade routes make low trade cities worth the effort to irrigate, road and build improvements in because the additional trade arrows from the trade routes will make cash if there are MKs, banks, and stock exchages built. There needs to be roads to generate more trade and irrigation to make more people to take advantage of the roads, temples, cathedrals and colliseums must be built to make the people content, libraries and universities are built to also take advantage of the trade, and so on and so forth.
So that's why trade routes are important. Secure trade routes for ongoing + trade first, then build freights/caravans for initial beaker/gold bonus.