A few months ago I picked up a new group of players, which then went on to form the Bear Company.
As they started to rampage around the Uplands and the Western League (like all my other groups) I needed to know what they would be comfortable with. The point is that between adults you can discuss these things, but I knew nothing about them, and a few never played before. So I needed a way to discuss which topic was ok to include in my gritty campaign without discussing them. So I sent them an email saying more or less:
my campaign setting is quite grim-realistic. There’s a lot of mud and pillage and historical materialism and proper real-history horror. I’d like to know how comfortable you are with violence, horror and all the horrible things that History books mention. I’m not sure how you people are comfortable with the current grimness level. Orphans and starvation and ogres eating people and mutilations and upper class people being horrible douchebags mean that the dial is set at about 7 out of 10. I’m fine with anything from 4 to 10 (yes, the dial can go up to 11, of course). But before grimness goes to potentially horribly triggery levels I’d like to know if this is too much or good enough or you are fine with trying more or less.
And then we had a short but productive email discussion where people brought forward things that they were not fine with (me included). Then we ended up playing a campaign with more weird than grim, but that’s in the scheme of things.
Anyway, the point is that you want to avoid alienating and hurting your friends/players, and between adults you can have civil conversations without hurting each other. But sometimes you can’t easily discuss this with strangers because you don’t know them.
Maybe even referencing some topics would be too uncomfortable for people, especially with people you don’t know.
The Grim Dial is not perfect, but it’s a handy metaphor to signal limits without discussing the topics themselves. You can assign topics to dial positions and anybody can say something like “I need something lighthearted, 2 or less, like Adventure Time but without kidnapping or postapocalypse undertones.” and someone else says “Yeah, 11 is fine, but no kids anywhere near weapons or sex at all”.
TL:DR: the Grim Dial is an abstracted handle on how grim/lighthearted your game want to be. It’s to be accessible to everybody at the table. It’s there to signal off-limit topics without even discussing them. It’s a safe way to introduce difficult topics.