Individual packets of data that traverse a network can be intercepted or monitored by anyone who can access the packets at any point along their path from sender to receiver. This means that if you want to exchange data securely over an insecure network, you need to use some form of encryption.
Cryptography is possible without a computer, but most often the information to be encrypted originates or at least ends up at some point in machine-readable form—a word processing document, ASCII, EBCDIC, or some other standard computer format. Thus, computers have given rise to the means by which information is most often encrypted, and have also provided a huge and growing population of potential users of encryption.