No reason at all. Don't worry about it.

Some people just like the firm anchor that the spikes appear to provide on really thick carpet. But as long as your speakers don't sway in the breeze, they're firm enough.
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Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
Sony PS3, surround backs
-Chris