Quite simply explained we use networks for communication between computers, sharing of data and peripherals. In the business world we use networks for ease of administration and to cut costs.
Sharing data example imagine an office with 5 secretaries working on 5 different computers, one requires a file from another computer in a non networked office this file would have to be written to a portable media then loaded onto the computer. In a networked office the file could be accessed via the network from a shared folder.
Sharing peripherals example the same office with 5 secretaries working on 5 different computers, in order to print their work each computer would need to have a printer attached. In a networked office you could have one shared printer, cutting costs