David,
It's simple. Some engines are designed to develop their ideal Horse Power at an RPM range that is simply too fast to be turning a propeller. To keep the propeller tips from going super-sonic and still allow the engine to develop the max Horse Power, you sometimes have to use a reduction drive. Most engines designed specifiaclly for aircraft are designed to make their HP at lower speeds by using more displacement, so they don't need to add the weight and complexity of a reduction drive, but many of the lighter aircraft engines or conversion engines will need to use a reduction drive as they may be running at 5,000 to 8,000 rpms at max power. Depending on the size of your propeller you will probably want to be turning at 2,500 to 2,800 rpms.