Sorry. I didn’t realize I had to give more evidence. 1e modules almost all had boxed text. Tomb of Horrors, one of the earliest modules has a picture gallery to show players. Until recently, setting guides were very, very popular books with hardcore fans who are dedicated to the canon of the setting.D&D’s popularity didn’t begin with Dragonlance. Sure adventure paths are popular. But that doesn’t make them essential features or core parts of the RPG experience. Adventure paths are not required in the least
On and on. Hundreds of pages in Dragon dedicated to the performance end of running a game. Endless player handouts and other goodies to use at the table. Entire libraries of gaming music.
For something that’s not essential, it sure has gotten a ton of attention over the years.