Why use ground wire from preamp to amp???

Arcticdeth - rprince is correct - they are using the ground lead to ensure upstream source components do not cause hum.

Case in point - I had a Pioneer ELITE DVD player and a NAIM amp.

NAIM’s "architecture" assumes that all SOURCES are correctly grounded to the mains ground

The Pioneer however was "isolated" - it only had a two pin plug - and when used with the NAIM caused a loud hum. Measuring voltage between the chassis of the Pioneer and the mains ground  with a multi-meter I was surprised to find a potential of 2 volts. 

Grounding the chassis eliminated the hum completely.

Since several of todays source components (like DACs or Phono Stages) come with either USB power or Wal-wart power supplies they can often be the source of "noise".

At one point in time - all of my sources had isolated power supplies - so I implemented my own chassis grounding scheme - just to ensure it was as quiet as I could get it. 

It does no harm to your components to implement this type of scheme - but if your components are correctly grounded it is not required. 

I prefer to connect each "isolated source" to a central "grounding point" that connects directly to the ground of the power supply and not "daisy chain" it via other components.

I now have source components that are properly grounded so I no longer use this grounding scheme

Regards...

I see no reason for the ground wire.

In the case of a piece of audio equipment that does not use the wall receptacle safety equipment ground, if the equipment was designed properly the signal ground of the piece of equipment is connected to the metal chassis of the equipment.

A piece of audio equipment where the equipment is designed to use the wall receptacle safety equipment ground the designer of the equipment may have or not have connected the circuit signal ground to the metal chassis of the equipment. Some designers incorporate a switch so the user can lift the signal ground from the wall safety equipment grounded chassis.

When an interconnect is connected between two pieces of equipment the signal grounds of the equipment is connected together by the signal ground return wire of the interconnect.

Any time a piece of equipment that uses the wall receptacle safety equipment ground, where the circuit signal ground of the equipment is connected to the metal chassis, and then connected to a piece of equipment that does not use the wall receptacle safety equipment ground, that  piece of equipment also then becomes grounded to the wall receptacle safety equipment ground by the signal ground wire of the interconnect.

I suggest you look for another interconnect manufacture.