For some drummers, it's great to have an IEM mix to (hopefully) hear everything on stage really clearly. One of the reasons this can be an issue is a "normal" position of the drum set in a band:
Obviously, if the amps or other instruments are pointed towards the audience, they're pointed away from the drummer. So, having a wedge or IEMs is pretty important. And, for louder music, the drummer will really need to hear the kick drum, and this requires low frequencies.
If using a wedge, you can add a sub. You can feel the vibrations from subs because they're physically pushing a lot of air to create low frequency sound waves. This can really add to the feel of the drums. And even though IEMs can have plenty of low end, they don't actually move air like subs do, so it's common for touring productions to do stereo IEMs plus a sub for the drummer.
However, sometimes having a sub for the drummers isn't ideal. This can be because of space concerns, or because too much rumble on stage can be disruptive for the FOH mix. But, there is an option that some drummers like. A "Buttkicker", first developed by the Guittamer company, clamps onto the drum throne and shakes it so you can feel the bass without using a sub.
This is how it works: the signal from the mic is sent to the console (either FOH or a dedicated monitor console). A mix is then sent back out of the console to an amplifier. The strong signal coming from the amplifier is strong enough to shake the butt kicker, which then shakes the throne so the drummer can feel it. Here's a diagram.
While some people believe that the vibrations aren't too great for your spine, the ability to really feel the kick drum is pretty cool. From Gretsch artist Jody Giachello: "I don't know if I prefer it over a sub, but it's definitely nice to be able to feel it in your body." And I agree with him.