It's like Van Halen and the brown M&Ms thing. It determines if the author of a C++ lexer, syntax highlighter, etc., read the Standard. Try splitting a digraph with \ and a newline.

> What is the purpose of this?

Its explained in the first two sentences of the page.

I like the use of "not" because it stands out more clearly than "!". "if (not (x and y))" seems more readable to me than "if (! (x && y))".