The Dungeon Master’s Guide (DMG) remains one of the most popular books for D&D Fifth Edition. Why though? You don’t actually need it; The basic rules and Player’s Handbook contain all the information you need to run the game. This is especially true if you are planning on running a published adventure or module. So why get a copy of the DMG?
The DMG is still one of the best sources for magical items. Tasha’s Cauldron of everything contains a lot of very good class-specific items. Xanathar’s guide is a great source for more utility focused and common magical items. Fizban’s has very powerful dragon-infused items. All that said, the DMG contains the core magical items that you are most likely to need for your players.
The Dungeon Master’s Guide and Worldbuilding
The DMG is an invaluable source for worldbuilding. Dungeons and Dragons has it’s own unique cosmogony that is helpful for DMs to understand when they construct a world. Things like astral travel and the Ethereal Plane, both of which players have the ability to access, are explained in the DMG. The section on creating a multiverse is indispensable for understanding how to understand and create alternate planes of existence for your game.
The DMG also provides guidance on how to create spells, weapons and villains for the game. It also has a deep dive on how pantheons of gods work, and how to construct them for your game. It is entirely possible to do this on your own, of course. However, the DMG provides you with a lot of guidance for how to get this done.
The DMG has a lot more to offer as well. I’m just pointing out the parts that have been most influential in my game.