If you want to play a game, you first need to know how. Nowadays, games have tutorial sections in them, but not so long ago games used to have manuals included with them and I don’t think that we should look at them as an outdated concept, that does not make sense today. On the contrary, I think that for most games, writing a manual is a better choice than making a tutorial section, because:
Of course, it does not matter that manual is better, if people won’t use it, so how many people want to read a manual? To answer that, let’s consider the Wiki websites. Most games have some sort of a wiki site, which are written by the players in their spare time. If you think about it, a wiki is basically the same thing as a manual. Some games, usually turn based strategies, even have a manual built into them. While I don’t have much in a way of data, the fact that Fandom.com has around 250,000 wikis (not all are games related though) tells me that there is a significant demand for this.
And finally, when is a tutorial actually needed? Well, when the game does something so weird, that it is hard or even impossible to describe on paper and it is necessary for the player to have a way of trying it out in a controlled environment in order to not have a bad time when starting the game.
Sometimes the developers may also want to omit things from the manual, to keep some surprises for the player, but I think that usually these are the kind of surprises that make you curse the developer and reload the last save, so they would be better off to prepare the player by explaining how to deal with it in the manual.