The difference between High and Low Bar

1. High Bar is placed high on the trapezius muscle across the top of the shoulders and for you to trains your quads. 

2. Low Bar is placed low on the upper back in the back squat position and for you to emphasizes your lower-back and glute muscles.

Also, both the high and low bar can improve balance and range of motion.

I think if you want to build power, the most effective way is high bar. On the other side, if you want to build muscle, the most effective way is low bar.