Its flexibility is beyond imagination. Programs can emit anything from simple numbers/vectors/matrices to medias (image, sound, video, either loaded or generated) to interactive programs, all of which can be embedded into the notebook. You can also manipulate every input and output code blocks programmatically, because it's Lisp, and can even programmatically generate notebooks. It can also do typesetting and generate presentation/PDF/HTML from notebooks.
What people have been doing w/ Markdown and Jupyter in recent years has been available in Mathematica since (at least) 1-2 decades ago. FOSS solutions still fall short, because they rely on static languages (relative to Lisp, of course).
I mean, really, it's a technological marble. It's just that it's barred behind an high price tag and limited to low core counts.