Because when your lights are too bright than they can limit your ability to see outside the lit up area. I do a lot of night wheeling in my Jeep and have lights underneath the rig to help see whats going on around and below the rig. If those light are too bright you can't see anything beyond the light worth a damn.
I've had to do many hours driving with just black out lights and moon light for the military and you would be amazed at how well you can see with little to no light. That's w/o NVG's too btw.
1998 KLR-650 (wrecked)
2000 KTM 950 Adventure (sold and regret it)
2001 KTM 640 Enduro/SuperMoto (sold, no regrets)
2008 Honda Shadow 750 Spirit
2011 Kawasaki Vulcan 1700 Vaquero