I first encountered this in Bjarne Stroustrup's 2000 book, _The C++ Programming Language_. As he notes in the introductory material:

In code examples, a proportional-width font is used for identifiers. … At first glance, this presentation style will seem “unnatural” to programmers accustomed to seeing code in constant-width fonts. However, proportional-width fonts are generally regarded as better than constant-width fonts for presentation of text. Using a proportional-width font also allows me to present code with fewer illogical line breaks.

I switched years ago and would never go back to monospace.