The balancing process creates equiality and strength in the target area, when using machines (be it a smith machine or any other) many other areas in the body become involved, many muscle groups can become involved when forcing an object through a resticted axis (ie. smith machine or general gym machines) therefore it takes a lot more balance (unilateral muscle involvement from both sides of the body) and coordination to move the same wait through a desired axis whilst having to mantain control, hence an amplified effect in the target area

i hope this helps