The pickup height is important to the output of a pickup. The closer the pickup is to the strings, the magnetism in the pickup will magnetize the strings stronger. As the strings move back and forth over the pickup, the moving magnetic field through the coils in the pickup generates an alternating current. Having the pickup closer to the strings will generate more current than a pickup that is further away. Having a pickup closer to the strings will make the pickup sound louder and fuller. Having a pickup further from the strings will make the pickup sound softer and less full. You will get a cleaner sound from pickups when they are adjusted lower.