Dungeons & Dragons is a game that combines tactical combat with interactive story-telling. As much of the game is free form, the results vary widely. And because the game is asymmetrical, the DM carries somewhat more responsibility than the other players for everybody having fun. While every DM is different in some aspects, there are a number of recognizable "schools" of the art of DMing. And one important distinction is the amount of preparation different DMs put in, which can vary from nothing at all to spending far more hours preparing than playing.

Personally I am at the more prepared end of the scale. I not only believe that preparing D&D adventures well gives a better result when playing; for me preparing is also an opportunity to spend more time with Dungeons & Dragons, seeing how there are frequently two or more weeks between my actual games of one campaign. I admit that some of my preparation is a bit over the top, like printing all the monsters with a 3D printer, or getting my battle maps printed in poster format. However the result of having maps, monster miniatures, and handouts is a far more visual game. That appeals to a lot of players, because the whole "theater of the mind" thing isn't really everybody's cup of tea. It doesn't make the game less imaginative, because the miniatures and maps are just visual aids, which don't keep players from imagining the situation in more detail in their heads.

If necessary I could play without those materials. But the preparation I consider to be absolutely necessary is knowing the adventure you are playing very well. It is the nature of the game that players will do surprising things, and if that reduces the DM to frantically paging his adventure module to try to find out how he should respond that really kills the flow of the game. Knowing your adventures well also allows the DM to foreshadow, dropping hints of things to come into the game which make the world feel so much more alive and less scripted.

Having said all that, being able to arrive at a good preparation result faster is an obvious advantage. Now that I am DMing a second campaign at a local RPG club in parallel to my "home campaign", I have the opportunity to use the oldest trick in the book to speed up preparation: Recycling. I am playing adventures in 5E which I previously played in 4E in my home campaign. Currently I am preparing Madness at Gardmore Abbey, an adventure I played in 4E back in 2013/2014. Having already played through the adventure for a whole year, I know the story very well. And I still have the files for the battlemaps, as well as flowcharts, handouts, and other preparatory materials. With me playing more, and 5E being a lot faster than 4E to play through the same adventure, getting the same preparation done is less time is certainly welcome!