The need for a manual varies with the type of equipment- I've never had a manual for a disc, or for a hay rake- they're pretty self explanatory, and not very complicated.
That said, IMHO, a baler is the one piece of equipment for which you MUST have a manual- because so many things can go wrong. Not tying correctly? A baler manual will have pictures of the different knot problems you encounter, and will tell you how to fix each one. And the other reason is the disastrous consequences of not adjusting it right. That plunger coming back will make real short work of about $2,000 worth of parts if its not timed correctly, and you can't time it "by feel"- you must do it correctly, or haying season is over for the year.
I've had 4 different balers over the years, and I have yet to go to the field with any of them before I got a manual. And I have never had a manual for many of my other implements, and am none the worse for it.