For me - obviously there's the convenience of having the whole amp in a single box. One less step to set up, and I'm rarely in gig settings where I need a bass stack.
But the other critical thing is that, some years ago, I started bi-amping, splitting an octave-up signal to send to a guitar amp. It helps a lot to fill in the frequency range when the guitarist solos (and it's just him, me and a drummer). I actually bought amps checking the width dimensions so that they could sit neatly one on top of the other...
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For bigger stages, and where there's more instruments so I don't need to fill under guitar solos, I sometimes bring a vertical 4x10 stack...
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