My vision of what the manual should do is this: It should be a place for beginners to start. It should be a reference for all the features in ScummVM. Of course it is a moving target, but some features that have been there for a long time are seriously underdocumented in the README. Sure, people can in theory find everything on the forums, but that is not always easy, and in a thread replying to such a question, there are often also many slightly incorrect to completely wrong answers. Sifting through all this is quite a task. Not to mention that you *have* to be online, while a manual could be converted to HTML/PDF and saved on your device.
Some examples of things that I think would be nice in the manual:
* Quick start guide: brief guide on how to add any game and start playing it
* Document all settings in the options dialog. And not just like this: "Aspect Ratio correction checkbox: When activated, this enables aspect ration correction"

. It really should explain what that means (old screens had non-square pixels, modern ones have square pixels, hence image looks squashed, so we stretch it), and mention limitation / which engines/games/devices this applies to (e.g. ARC applies only to 320x200 games).
* It may also link to other places (forum posts, wikipedia, ScummVM wiki pages, ...) which provide more details; but it should try to be self-contained at least for the basics, so that is useful even when online.
* Some people like to print instructions, and that's not possible with our forums (resp. would require quite a lot of paper).