The Ground Control has the offset to compensate for the differences in MIDI program change commands, but that isn't all it does. The Ground Control also sends commands on individual MIDI channels for up to 8 devices,(at least on my older model, newer ones may accommodate up to 16 devices). This way when you set up your patch changes, you assign the program presets to each device as you are programming the patch, then save it in a location on the Ground Control.
It's much simpler than MIDI mapping, and much faster.
Plus, the Ground Control also allows you to plug in two passive volume pedals into it, and assign these as MIDI Continuous Control to any device that can accept it. It's a much more powerful tool than just a simple program change pedal.
Used in conjunction with the GCX Switcher, allows the Ground Control to bring non-MIDI devices in and out of the signal chain, up to 8 devices again.