Why you use mirrors as reflectors and not alluminium?

Nearly all modern mirrors are aluminum. An aluminum coating is placed on a sheet of polished glass. In common mirrors we observe this aluminum reflector through the glass layer. But if we turn the mirror around, we can bounce light directly off the aluminum, and this is called a "front surface mirror." (Common mirrors have a coating of hard paint over the thin delicate aluminum coating to prevent scratches.) It's perfectly possible to form a mirror by grinding and polishing a thick aluminum slab. But aluminum is softer than glass, and would collect scratches and dings. Also, aluminum is expensive, so an aluminum mirror costing several dollars might replace a glass mirror costing a few cents.