As '79 said..

Acts as a damper for the sound.. gives it more of a thud than a boom.. Pretty much every drummer I know has something in their kick drum.. I've also seen them stick damper rings or mufflers on their toms to get rid of some of the ring.. you want drums to punch you in the chest, not boom up the whole mix and make it muddy.. but, I'm not a drummer, s I probably just totally misinformed you lol

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