Answer:
Everything in your computer requires power. The power supply has leads running to the motherboard to support CPUs, PCI slots and their respective devices, USB ports and optical drives.
#2 ---
A power supply does not really supply power in the sense of a reservoir, it is actually a power converter. Light bulbs and heaters use the power coming out of your wall, but there is no electronic gadget that uses power directly from the wall. The 110 volts AC (alternating polarity) power must be converted to voltage levels that the circuits are designed for, and usually at low voltage 5 or 12 volts DC (steady direct polarity). There is a huge industry that builds power supplies that do this.