Overdrive guitar pedals emulate that sound of an overdriven tube guitar amplifier. Often found in rock, blues and country music. Tube amps can cost a mint. Although most guitarist aspire to one day have a tube amp, an overdrive pedal can offer a similar sound for a fraction of the price.
In fact, the MOEN UL-TD pedal features a real tube that you can overdrive. The Daphon E20OD has the classic overdrive sound made famous by the Ibanez tubescreamer. It is actually said that the E20OD has the exact same circuit chip as the tubescreamer and deliver basically identical sounds.