I think tonally for me an onboard eq really opens up my possiblities. Being able to cut and boost the bass, mids, and highs on the fly is important for me in getting the right sound/texture. Rather than running back to the amp to adjust, it's great having that flexibility at your fingertips. I play a wide range of styles throughout a show and to have to make all those adjustments on the amp would be a pain in the ***. There's quite a bit you can do with a tone knob, but having the preamp for me is much more versatile. That's just my personal experiences/preference regarding an onboard preamp.