As far as I can remember, it was something they used while developing MK9 to check some stuff with stances.
Figured out they had one button left unassigned, gave it to the stance change.
It had its uses in MK9, Scorpion used it to bait out jumpers who were expecting hellfire.
In Injustice I'm using it with Nightwing in his Staff stance, taunts them a bit and makes the opponent think I'm doing something.