Manuals existed because developers didn't understand how to make such games that it's clear to the gamer what needs to be done. If the controls, signals and whole game world are rational enough, no manual or ingametips are required. Only controls need to be explained.
Actually i have to disagree, baldur's gate and baldur's gate 2 i knew what to do almost as soon as i started (low learning curve) and the controls were easy enough to figure out without the manual but those manuals were crafted with love (especially BG2's) and made me love the game all the more, i wish the games still had manuals that were as great as bg2's.
F.e. Might and Magic series. Without manual it was hell of a fun but with it the game turned into science and I practically planned my every move six steps ahead.