I’ll first clarify one thing and then explain why there’s this limitation.♥

Ah, for “wallpaper” stuff you can make up stuff as much as you want! They’re like “what color is her towel” and you go “it’s a light yellow”. Maybe your notes say “The desk is full of uninportant papers. Hidden under the rug is a key” and the players start looking at papers and you can start making up geneologies and transaction accounts to your hearts contempt. That stuff happens all the time, room descriptions should be short & sweet knowing that you can make this sort of thing up.

It’s only for “salient” things—the core main things the players and their characters are interacting with—that there’s a limit. For example, the other day I ran a module that said “the desk is full of important documents. GM’s choice”. That violated this principle. If the documents are so important, they shoulda been specified before the session began. (It’d had been OK if the module had said upfront that “DMs, you need to specify the documents on page 277” but it did not.

Now on to why I personally think it works better to limit improvisation in this way! I used to be a 100% improvisational GM. That came at the expense of player agency. They go west, they meet whatever is at the top of my dome at that point. They go east, same thing. Every direction will result in GM’s whim. Then I had some experience with a more prepped style, tried it, and it was awesome. For example, if you’ve prepped “if they go west it’s a dragon (young green dragon) and if they go east it’s a shop for minotaurs. 2d4 minotaurs are there. They wanna buy wax but the shopkeeper (human commoner) is all out of wax”.
Suddenly it matters whether the players go east or west! Fun fun fun.♥ There’s still room for some improvisation, you can still improvise things like the description of the shop, and over time you’ll also build up a toolbox of default rules & generators (that specific prep can override), like you might have rules for what a typical shop will carry and how often they restock etc.

Also if they spend a lot of time on their characters, let’s say one character gets a high lockpicking skill and another character gets good at fighting djinns and efreets, it wouldn’t feel fair if I then were to make all the locks extra tough “to balance it” or make all the efreets extra feisty etc. And it would definitively not be fair if I were to improvise something like “20 angry beholders show up”. If I instead stick with the prep I can run the dungeons and fights fairly and toughly without fudging. (That’s not a comment on fudging generally, it’s just that in this particular playstyle it’s especially disallowed.) Their character making choices will start to really matter.

There’s also the matter of “encounter balance”. That’s something I don’t have to worry about; I can compare monsters to other monsters and generally put the more dangerous monsters further away or deeper down, and then use XP leveling instead of milestone, and then players will tend to send their characters to an area where the danger level is good for them; enough XP rewards to make it worth it but not too dangerous that they won’t have a chance. Sometimes they steamroll the monsters and other times they die; it’s less that a specific encounter is a challenge and more that the entire world is the challenge. Or is their playground, I should say; if they just wanna set up shop and run an inn or something, that’s fine too.