Although the client-server idea can be used by programs within a single computer, it is more important in a network. In a network, the client-server model provides a convenient way to interconnect programs that are distributed efficiently across different locations.
Computer transactions using the client-server model are very common; for example, to check a bank account from a computer, a client program in the computer forwards the request to a server program at the bank. That program may in turn forward the request to its own client program that sends a request to a database server at another bank computer to retrieve the account balance.
The balance is returned back to the bank data client, which in turn serves it back to the client in the personal computer, which displays the information requested.