It's similar to the appeal of LaTeX: you can write macros (ordinary Racket functions) to do common things. I could build on Scribble to make an online textbook, for example, with cross-referenced theorems and definitions, by defining some extra Scribble functions.

Except, unlike LaTeX, you have a sane programming language instead of hairy macros.

Pollen takes Scribble and strips out much of the structure, leaving a very interesting core: http://docs.racket-lang.org/pollen/