A ground wire kit is used so that you can have better grounding i.e less resistance on your electrical circuit. As you probably know most electrical circuit takes the -ve from your chassis and sometimes from one point to another it might have some resistance, using such a kit will reduce the electrical path for the negative track and give you less resistance.
Conclusion
That might help I have read in the forums that people with the kit installed have brighter head light, blinkers fog etc ...