Dust from sanding, cutting and grinding concrete, stone, fiberglass and drywall is extremely fine and extremely hazardous. These tiny particles can go straight through typical shop vac filters. Watch the cloud of dust that's coughed out the exhaust port when you turn on a typical shop vac. That does not happen when using a Dustless Wet Dry Vacuum. The optional Micro Pre-filter is NOT just another vacuum bag filter. It captures particles down to 0.5 microns - incredibly close to a HEPA filter that captures particles down to 0.3 microns. That's important because the Micro Pre-filter is an integral part of the patented filter system. It fits inside the main canister and is designed to hold as much as 60 lbs. of dust, dirt and debris. It also retains its strength when wet. The Micro Pre-filter helps keep the primary and secondary filters clean, extending their life. Other vacuum systems lose suction after filling with just a few pounds of dust.