They were crucial in more than one way! Quite a few games used manuals as a simple form of copy protection: you had to answer some question at start that required the manual to answer. Sometimes explicitly so, like asking for the first word on some page.
Sometimes it was more creative. E.g. people who remember F-19 / F-117 flight sims might also remember how on startup you had to pass an "aircraft identification exam" - given an image, guess what it is. And if you got that wrong, you could only fly training missions. Of course, this one didn't strictly require the manual - you could learn from playing the game itself, or you could get that info elsewhere. I wonder how many people still remember that kind of arcane knowledge just because that was the game they played a lot as a kid and eventually just memorized all the answers.
Sometimes it was more creative. E.g. people who remember F-19 / F-117 flight sims might also remember how on startup you had to pass an "aircraft identification exam" - given an image, guess what it is. And if you got that wrong, you could only fly training missions. Of course, this one didn't strictly require the manual - you could learn from playing the game itself, or you could get that info elsewhere. I wonder how many people still remember that kind of arcane knowledge just because that was the game they played a lot as a kid and eventually just memorized all the answers.