Just a note: Markets do not create extra commerce

, they create extra gold

(the icon is very similar and easy to confuse). If you convert commerce to gold via the slider then yes markets are useful and are producing extra gold for your empire.
Binary economy; When you run at either 100% commerce or 100% science on the slider makes more use of your Commerce buildings
it is explained in the illustrated guide to economy and plenty of other places.
If you are running the slider at 100% science, ie. converting commerce into beakers. Then your markets are doing very little. (a common scenario) Non commerce sources of gold that I am aware of are: Merchant specialists, Shrines, Corps.
Priest specialists, Harbours
Markets do also produce extra happiness with certain resources and allow for 2 merchant specialists and since it is common for the slider to stay at 100% research this is often their main usefulness and since the market is an expensive building there are often better alternatives.
I personally find markets to be rarely useful. They can be good in a decent shrine city or corporation HQ. I will occasionally build one for the happiness in my capital (all other cities would be whipped instead) It is extremely rare to build one for Merchant slots but not impossible I suppose.