The reason is because the engines produce this material as a waste product. So instead of venting it into space it's processed and funneled back through the ship to power everything from lights to equipment.
Very efficient and very VERY dangerous. Many Vulcans retired from the VSA because Humans pulled shit like this.
"Inertial dampeners have failed."
Many times someone will say this while the ships is performing combat maneuvers at several hundred kilometers per second.
If that were true, everyone onboard would instantly become "chunky salsa." (Obscure Trek-related quote, for anyone that can place it.)