Onboard controls and a buffered low-impedance output (which may give you a tad more "fidelity" thru long cables). FWIW.Forgive me, but what will the onboard pre provide that a quality, versatile, front end of a modern amp won't?
Onboard vs outboard preamp is of course its own hot topic. (I personally prefer having the onboard preamp for "modern" sounds over trying to "activate" a passive bass with an external unit or at the amp - YMMV.)