I assumed it was for people writing "polyglot" toy programs, because it'd be very useful resource.
I very rarely encounter uses of the term "polyglot" referring to people knowing large number of programming languages, so I didn't think of this use. Looking at it now, from this perspective, I have to say that I don't think it would be useful for these people, not enough depth.
It isn't so in this case, as the code for each language is clearly different. It's more of a 'construct translator' kind of thing.
It's always the case. That's why writing them is interesting.
Which would come incredibly useful if you wanted to write a polyglot program.
Why else would you use a reference like this? If you wanted to learn a language, or had to hack something in a language you don't know, you'll be better off using resources specific to given language(s).