Why put more than one winding on the low voltage side of a transformer?
Any given magnetic core of a certain area at a certain frequency will only generate so many volts per turn. 

For instance, an iron core 12x25mm cross section is only good for about 0.1v rms per turn at 50Hz, needing 2400 turns for 240v mains.

You could in theory make a core with enough cross section to give you any number of volts per turn, but that would make for a very expensive and heavy transformer. It's cheaper and lighter to use less core area, and more turns, when making a small transformer, to match the voltages you want to use.