That big red mushroom switch is the last chance panic button. IF you are there, and you see that everything is going south, a push of the Estop should stop all axis movement.
If you have the spindle controlled through Mach3, Estop will also halt that.
I personally, like to HAVE that Emergency Stop button handy.
THT: I cannot answer anything about EMC2. I'm a Mach man.
CR.