You start getting nasty events in provinces which don't contain at least previous available level trade building, or don't border one which does. So if your tech allows to build Seasonal Markets, you need each of your provinces to at least border one with at least Bi-Annual Fair.Who the hell controls Canton?
Also, Trade buildings (Bi-Annual Fairs, Seasonal Fairs, Bazaars, etc) increase marginally the price of your trade goods, stabilitiy and your population growth while slightly decreasing taxes. At their higher levels they give % increases to your Trade and Production Efficiency. Also, if you spam them, your merchant faction starts to like you a lot more and your noble/bureaucrat factions like you less.