Why are there digraphs in C and C++?

Digraphs were created for programmers that didn't have a keyboard which supported the ISO 646 character set.

http://en.wikipedia.org/wiki/C_trigraph

I believe that their existence can be traced back to the possibility that somewhere, somebody is using a compiler with an operating system whose character set is so archaic that it doesn't necessarily have all the characters that C or C++ need to express the whole language.

Also, it makes for good entries in the IOCCC.

I think it's because some of the keyboards on this planet might not have keys like '#' and '{'.