They were originally meant for use with reduction drives - gearboxes. Before the brushless motor and the inrunner/outrunner technology distinction made its way into RC flight, electric brushed motors were mostly of the high-revving low-torque variety. A gearbox was sometimes employed to turn that around, and a "slow fly" prop was more suitable than the thin bladed variety.
Nowadays, with gearboxes essentially consigned to history by the brushless outrunner, the line is blurred since a motor can be wound to produce just about any combination of kv and torque. Slow fly props give more thrust at a given rpm, at the expense of greater stress and current draw. Whether they're worth it depends on the application. There are no absolute rules (except "don't smoke your motor"