I use coated heads on all my drums including the kick, so I needed to buy something to get rid of some of the overtones for recording. This thing is a miracle honestly, it's so weird how well it works.
It comes with pre-cut segments so you can rip off the excess foam to make it fit to your particular kick drum - there's even a pamphlet telling you which cuts are for which kick sizes.
Really does make a difference, I even use this for my live playing as it seems to really tighten up my kick sound and makes it punchier without removing too much tone. Always better than stuffing hoodies and coats inside the kick anyway!