Mearls: The DMG is, well - going back to Basic D&D as a starting point - if you think of the Player's Handbook as for the player who is looking at character classes and played a couple of them and wants more options or wants to fine-tune what their character is, or who says "I want to play a paladin." The DMG serves the same role for the DM. Basic D&D hits core fantasy, it's stereotypical fantasy adventuring. If you're the DM and you want to do something more exotic, you say "I want to add technology to my game" or "I want to have more detailed rules for a grim and grittier game, more of a horror game." That's where the DMG comes in, it's for really fine-tuning your campaign, and creating a different type of experience than your standard fantasy campaign. It's also for expanding the scope of the game. So we've talked about things like ruling a domain or things like that. The more detailed rules for that would be in the DMG. We've talked about having some basic rules for things like that in Basic D&D but we're not 100% into it either way - is it confusing to new players or is it nice that it gives them a clear progression? We're still not quite decided on that yet. It's for if you want more depth on specific topics.
The DMG also has a lot of utilities in it, like for dungeon creation, adventure creation, creating monsters, creating spells, even if you wanted to create a character class. It's not quite the point-buy system from 2nd Edition, but it does say things like "Well if you want to create a class for your campaign then here's a good way to approach it."
So it's really for getting under the hood of how the system works and building up your campaign.
Bolding: So really, besides maybe Unearthed Arcana, there's never really been a hacker's guide, as it were, for D&D.
Mearls: No, exactly. And that's what we were inspired by. People like to tinker with their campaigns, and especially if you've been DMing for a while and you kind of want to do something different. Really going into in-depth [changes]. And now, it's not going to be deconstructing everything, but it's giving you the tools you need to make your own changes. And there's always going to be art to it, like monster creation, we can't give you a formula that's perfect. What do you do with a monster that has one hit point, one AC, and can cast harm once per day? How do you balance that? There's no simple answer, but even just telling DMs that helps.
I'm actually pretty happy to see this. Others may disagree with me, but I've found the advice for DMs in post-Gygax-era D&D to be of questionable value. ("Here's some problem players you may run into and some passive-aggressive methods for dealing with them, since expecting you to act like adults never occurred to us!") Getting at what the rules are trying to do and how tinkering with them might affect things would have been great advice back at the beginning. Having these sorts of assumptions and ideals spelled out in advance will give new DMs a leg-up on understanding what the game's about and what they'll need to do to make it be about something else.