I guess the idea is to distribute the pressure evenly over the bottom of the tank. I'm not sure it's always a good idea, however. I think the All-Glass tanks are designed to support the tank along the edges, via the frame. The center of the bottoms don't even "touch down."
For a
bare bottom tank, it makes more sense to me, but I just went ahead and placed my All-Glass 125 directly on the wood counter top on my steel bench.
Good idea to check construction sites, fishgirl. Seems the "home improvement" stores are hit and miss. They don't stock it around here.