The bonuses of markets add up since. Better markets will allow further development of other infrastructure such as aquaeducts. The increase in population due to this will increase trade income and tax income further... Markets also give some good retinue such as the silk merchant (+10 or 20 percent trade). After all, a market is a long term investment into the further development of the settlement. Roads, mines and ports have higher priority, however.

Schools will give your characters valuable ancillaries, increasing especially your character's effectivity as governor. Having a character with an mining engineer +20 or 25 % mine income) can really boost your income if he is placed in a province with 4500 or 3000 mnai mine income. Schools are also necessary for most historical ancillaries, but those give mostly rather small bonuses.