did the grounding on mine when installed a more powerfull sound system on mine,

mine starts at the left hand side strut brace bolts which goes to rocker cover then to inlet mani,to alternater and then to the car chassis again where battery negative goes and also put an extra cable from battery negative to chassis. use some old 4 gauge i ahd over.
 
what is the main benefit of this???

There's a few theories I've found so far;

1. Provide an equal ground to all parts of the car so signals from air flow meters, lambda sensors etc are cleaner and behave better, and possibly giving better performance. That's how they're sold, in fact even that groundingkits.co.uk use a version of that.
2. Improve the quality of sound systems, which I guess is what M87NER is after.
3. Clear up the grounding of an old banger of a car, so that whenever an MP3 player is plugged in the driver can listen to music instead of awful alternator noise. This is me by the way... :)

It is one of those things that sounds quite plausible, and it's cheap enough to 'have a go', and if it doesn't make a difference then no great loss.

A quote from that website for you;

This always creates a lot of debate - the generally accepted theory is that installation of an earthing kit can restore lost power/torque, where the loss is due to inefficient earthing, due to age, wear & tear etc. You might not realise your car is running short on power because of an earthing issue, but when a grounding kit fixes this you'll notice.