Individual, or split brakes allow for a very tight turn when only one brake is applied. The tractor can literally spin in one place. Also, and probably more useful for most applications, when using heavy ground engaging equipment, sometimes the front wheels can have a very hard time steering the tractor, due to the greater forces happening in the back. Split brakes will allow the operator to "brake steer" and keep the tractor and implements moving in the right direction.
Hope this helps.
Joe