I think I have read that it boils down to two things:
Firstly that a good support reduces/prevents/counteracts/ vibrations, thus allowing the equipment to work in a nice uninterupted way.
Secondly, that supports made from different materials can affect the overall sound of the equipment. I have read that some glass supports can make the sound brighter, whilst wooden supports have a fuller more mellow sound....and all that.
I can understand the vibrations part to a point, especially where vinyl is concerned, but overall I personaly think its all a bit suss. As for changing the sound of the equipment based on the supports material.........well Im not subscribing to that at all.
The only time this may happen IMO is when the support it'self begins to vibrate and produce a "ringing" sound, I have heard this on a cheaper metal speaker stands (easily solved with some sand etc in the stand).
To me all a stand needs to do is support the equipment in a secure way, be well built (cos its costing me cash) and allow for good ventilation.
How some of these "hifi" company's justify the costs of their supports is a mystery to me, granted some of them are very well made and good looking, but the prices!!!
anyway i'll shut up now