Treadmill mats serve a number of purposes, one of which is to reduce noise. In addition, treadmill mats help preserve the surface upon which your treadmill rests. In addition to cushioning the surface of your workout area, the mat also collects motor oil, dust from the friction of the belt's movement and sweat.
The thickness of the mat impacts how much it can mitigate the noise of the motor running and pounding of your feet. The thicker the mat, the more noise it absorbs and the less pounding occurs on the supporting surface under the mat. This can be a significant benefit if you're living in an apartment, work out early in your family's house or simply need to shield others from your regular workout.
The Difference From Carpet
Don't think that a cushioned surface is equivalent to a carpet, which contains dust and, occasionally, long shag. Both are potential hazards to the long-term viability of your treadmill. Your treadmill's belt rotates, sucking in surrounding dust and potentially the carpet itself, if the shag is long enough to reach to the lip of the belt intake. The dust can shorten the life of the treadmill's motor.
Treadmill mats come in a variety of thicknesses and styles. The most basic is a thin, 1/4-inch mat made of vinyl. This type protects the surface, but doesn't provide significant sound buffering. Thicker mats come in two main types -- those that are one, elongated mat and those that are interlocking.
When purchasing, make sure the length of the mat or pieces to lock together extend beyond the length of the machine when fully extended. Check the size of the treadmill, not just the deck length. The deck extends from the engine housed in the front and adds to the total length of the treadmill.