Earlier quoted context omitted.
I remember reading a funny story somewhere recently that back in the day, ms developers intentionally left gaps in the docs or straight up didn't document certain apis etc so they could put them in their books and sell those so if you were working on certain things you'd be forced to buy said books.
I remember a different reason - APIs were intentionally not documented so Microsoft Products were the only apps that would know how to use them (and gain an advantage over the competition)
(Disclaimer: I speak for myself, based on my memory of public knowledge.)