Why Single Supply?
There are a few different terms used to refer to a system in which the designer has access to positive and negative voltage rails: bipolar, symmetrical, dual-supply, split-supply. Analog circuits are more straightforward and more mathematically coherent when a signal can actually go below ground.
However, the inescapable fact is that the dual-supply system is usually unpopular in the world of modern electronics. The reason is because to generate a negative voltage supply requires additional circuitry, which means more design time, higher cost, and a larger PCB; thus, if system requirements can somehow be met without recourse to a negative supply rail, all the better. The alternative to additional circuitry is a second battery; besides being applicable only to battery-powered equipment, this approach still introduces cost and bulkiness that could be eliminated by means of clever single-supply circuit design.
Of course, there is no law stating that a dual-supply system must have positive and negative supply voltages that are equal in magnitude. However, symmetrical supplies are the norm with amplifier circuits, and a discussion of dual-supply or split-supply systems may include the assumption that the supply voltages are symmetrical.