Why do you need a power supply?

Power supplies are need by many electronic devices for several reasons. Most devices need one or more DC voltages. Only AC power is available at a common household outlet.

Power supplies, among other things, usually must convert AC power to DC power.
AC = Alternating Current
DC = Direct Current
VDC = Volts of Direct Current

Computers, for example, need a power supply to convert 120 VAC line power to 6 VDC, 9 VDC and possibly 12 VDC. You will understand this more fully once you get further into the study of things like trigonometry, the calculus, basic circuit design, etc.