The Power Supply converts the alternating current line (AC), which is to strong for the socket, into the direct current (DC). At the same time it regulates the voltage so that it is not to strong for the sockets.
In the past you had to turn a computer on or off with a toggle switch. But several years ago the added a capability to the Power Supply, that permits to turn the computer on with a little push button and to turn it off with a menu option.
The Power Supply is one of the most important things in a computer. Because if you put another part of the computer in a socket, it would burn, because the current is much too strong for the socket.
All parts of the computer have to be conneced to the power supply.