Reduction gears are mainly found on turboprop engines. The power turbine spins at over 45,000 RPMs, so it is necessary to slow the process down a bit before you connect a prop shaft to it. The reduction gearbox allows the prop to turn at anywhere from 900 to 1,700 RPMs. It also gives you lots of tourque when gearing from 45,000 RPMs to 1,200. Jet engines use reduction gears to run various accessories, but they are not quite as essential as the turboprop's gear box.