555 timer and biasing
"biasing" isn't really the right word here; this is the IC's power supply. All ICs require at least two voltage levels to operate, including the 555; in the case of the 555 these are called +V and GND, or Vcc and GND, or Vs and GND, you get the idea.

The reason some chips get powered with both a positive and negative voltage with respect to ground is so that they can, without needing any complicated power circuitry, output voltages that are both positive and negative with respect to ground; it's easy to output any voltage between your voltage rails and difficult to output anything outside of that range.