THEY are actually useful for moving villagers. Put the villager in a normal mincart and put the furnace minecart behind it and right click on the furnace with coal
it useful when you don't have or don't want to use the recorses to make powered rails (let's face it they are pretty expensive[you need a lot of gold])
This is actually very very useful,if u put coal in that furnace the mine cart will start moving by being powered by the furnace,so its less expensive then using powered rails