yes they can be useful if your amp dies halfway through a gig, rather than carting a spare amp about, you simply connect the DI box to the desk and get to the end of the gig. Some like mine also can be used as a valve pre amp, to give you some warmth if your main bass amp is solid state and also if you play a bigger venue with front of house PA and decent stage monitoring and you don't want to lug your amp and cab to the venue, you can just use the DI box, great if your one band amongst many and you only have a short slot!