I actually have three, and I use them all--one for taking to yoga class, one underneath my gazelle to keep it from moving, and one on my hardwood floor where I exercise. I used to use one even on carpet because it kept my feet from sliding out from under me on my relatively slippery carpet,especially when I worked out in socks.
I wouldn't say that they're necessary, though. Just personal preference, to add padding or to reduce slippage.
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