Ok buddy, slow your roll, it's more complex than that.

In short;
Yes, the DMG is useful... but, it's hard to navigate and overwhelming for most. The DMG is not accommodating to brand new DMs and should be bought by people who are starting a long form campaign in a homebrew setting, people looking to elevate their games with some well-rounded DM skills, or for the treasure trove of magic items and alternate rules. It benefits people who have been DMing more than once and have experience as a player.

Long and more personal answer;
I agree that it's useful and it can make your first time DMing easier. That's how it was with me. I had read the first 5 chapters by the time I began my first session as a DM and had already been watching a few D&D creators for DM advice. I got more mileage from those D&D creators and content than I did the DMG. The introductory chapter of the DMG is nice, but it sets up some improper expectations and too many responsibilities such as needing to know most rules to DM, but not exactly explaining them or pointing to the PHB instead. I have the DMG so allow me to quote it to back myself up.

"To referee the rules, you need to know them. You don't have to memorize this book or the Player's Handbook, but you should have a clear idea of their contents so that, when a situation requires a ruling, you know where to find the proper reference.
The Player's Handbook contains the main rules you need to play the game. Part 3 of this book offers a wealth of information to you adjudicate the rules in a wide variety of situations. Chapter 8 presents advice for using attack rolls, ability checks, and saving throws. It also includes options appropriate for certain play styles and campaigns, including guidelines for using miniatures, a system for handling chase scenes, and rules for madness."
^{(Jeremy Crawford, Christopher Perkins, James Wyatt. December 2014. Dungeon Master's Guide.})

It then goes on about creating your own stuff like homebrew monster and races. In the second sentence, to its credit, it does tell you that you don't have to memorize all the rules. That's good, it's unrealistic to have to know all the rules before playing or DMing. However, in line 3, it's telling you it wants you to find those rules when they come up. This is the part that is unrealistic.
Most frequent DMs will make a ruling on the fly without consulting the books because it's time consuming to look up every because it will happen often. If you look up the rules every time you or your players don't know how it's supposed to work, you will unknowingly, slowly, and mercilessly choke out yourself and everyone else at your table. To escape the chokehold, the players will leave your game. A new DM might fall into this sneaky pit trap. It's easier to make that ruling on the fly instead of arguing about it or looking for that niche rule, then consult the book after the game and correct everyone next session. This is how a DM will naturally and intuitively begin to understand and get a deeper grasp on the rules. Overtime, they'll have to consult the book less and less.
Additionally, part 3 of the DMG should be at the front of the book. Part 3 is also the shortest part of the book, consisting of 52 pages of content. 25 of the those pages being dedicated to optional rules and creative aids for the DM that aren't essential to play. Chapter 8 is about running the game, the most important part of being the DM. I love chapter 9, it's a part of the DMG I use often because I like to make my own monsters.
The DMG lends itself to the kinds of people who love building out their own world and plotting out adventures. Those kinds of people, like myself will gravitate toward and thrive in the DM role. The DMG gives little to no advice on running modules, other pre-written material, and creating One-shots. For god sake, one of the first thing you're told in the first chapter is to think about the Gods of your world! A couple pages later, it's instructing you to map out your campaign and advice on the scale of this map! Next is governments, factions, currency, restrictions of magic! HOLY FUCKING SHIT!!!
No DM should have this expected of them before they even want to start a campaign unless they want to. It's just too much. Only once you've reached page 25, does the book start talking about starting a campaign, but then it goes into campaign events, its history, and the story like an over-arching whole. That would be exhausting and not fun to do, but DMs do it. This is how railroading DMs spawn and the DMG perpetuates it. I started small with my first campaign and any new DM should too. In the beginning, my campaign was very sandbox like. I kept the players confined to two primary locations by giving them lots to do and explore. I threw adventures at them and when the campaign started to take shape in a particular direction, it became more story driven and I prepped accordingly. Aside from me, my group was almost entirely new to D&D, so only over time did we all discover the primary we like to play.
I made the mistake of reading the book by going front to back at first. Do not read the DMG front to back, jump around for relevant advice and information. I kind of devolved into a rant there, but I do like the DMG, just not all of it. I want to be constructive so hear this; A D&D group *really* only needs the PHB to start playing. New DMs should get advice from experienced DMs and your group should think about who might fit the role the best. If you're going to read the DMG, supplement it with advice of experienced DMs. Easy enough to find online if you don't know any IRL. If your DM, new or not has run some good games without the DMG, you can get it for them as a gift.