A 8x2 grid of buttons can be used as a one octave keyboard. Space and buttons are scarce resources in grooveboxes, and grids of buttons are more versatile. You can always connect an external keyboard too, which will be much higher-quality than anything a groovebox can provide.

The deluge has a really clever keyboard mode that I find really musical to play with, especially coming as a guitar player as it stacks rows in intervals of a 4th

You can't really beat inMusic on quality at the same price as their various brands' keyboards. A button grid though is useful and pretty easy to manufacture.

Workflow and accessibility. Not to be reductive about it, but they're basically a massively reduced barrier to entry for tasting what 'real' production is like for the less musically inclined consumer.

Grooveboxes are essentially clip-launchers; they were basically incepted as a sort of sample-less Akai MPC utilising a built-in ROMpler and some basic Synthesis, some FX, and 'contemporary' presets to lure in people fiddling around in music stores - e.g. the MC-303 or to take it to its absurd conclusion, the DJ-XII.

The interactivity thereafter is about launching clips on the fly and fiddling with the resonance and cut-off for a lead sequence, and riding that master FX ping-pong delay like your life depended on it. The Dunning-Kruger DAW if you will.

The keyboard takes away space you could be using for more buttons.

The new Akai does.