Uses of computer networks how do they work in office?

Say a school for instance goes out and buys 20 brand new computers. Their hardly going to buy 20 printers, one for each computer now are they...?? They one printer and network all the computers to use one printer.

Better security. Its easier to keep employees away from undesirable sites, restrict what they can and cant do, watch what they do etc....

Share an internet connection... no point in buying a connection for every computer.

File sharing... if the boss wants the secretary to get a file, but is the lazy sort who doesn't want to have to physically get up off his chair and walk into the next room to give it to her. He can just just set-up file sharing, then call her from his desk and tell her theres a new file in the shared folder.