What's the use of ground coupling on digital oscilloscopes?
A switch which DC-couples the scope input to ground could reveal whether there is an offset in the ADC which shows up as a displacement from the marker where ground should be.

The switch also disconnects the probe's signal (it will not simply short out the probe!), so you can use it as an off switch break the probe from the circuit without disconnecting it. It's possible that the probe's ground clip is also cut when the input is in that state, which is even better since it is complete isolation.

If you're probing some node which is supposed to be at ground, the switch can give you a quick A/B comparison to see how close that node is to ground. If the flatline jumps slightly, it's a bit more visually obvious than comparing against the marker.