I'm a session prep guy. My prep scale runs from reviewing and updating fronts in Apocalypse World (and some PBTA games) to building those set-piece encounters in D&D(OSE) or making sure every single NPC has at least one belief and one instinct in Burning Wheel games. Prepping means putting my cards on the table so I don't cheat later or even need to collapse any quantum waveforms of setting. I've played enough games to know you cannot predict when those "magical drama" moments will happen. When they do happen, I want to be true to the game's mechanics, my prep, and then established fiction --in that order.
Three adventurers make a trek to newly discovered ruins, hoping to pick the best treasures before everyone else gets there. I prep this site the week of the session using a Dyson Logos map and various lore, journey, and faction generators for where I don't have an opinion. The Biggest Baddest Monster on the site is a mummy...and of course, they find it. Every player choice, every dice roll, every option, I'm there helping and reminding the players because I'm a fan of the characters —and it's easy because the prep work is set, as are the game's mechanics. I don't have an outcome to serve, or a waveform to collapse, no story to tell. The most experienced player, the party leader, figures they cannot win this fight, and they opt to run. I laid out what that looks like rules-wise; it's doable; they have a better movement rate over the mummy but little room for error. We move from fiction to my prep and then to resolution via game mechanics. It is more important, IMO to deliver a good game experience than to deliver a good story. From where I GM, we gathered to play a game. The story will be there when the last dice fall.
These three adventures all died separately and lost as they fled the mummy. Every roll, every choice since they broke open the sarcophagus was tense. In those last moments, we're playing the game, using every rule, every option we can leverage...engaging the mechanics to survive the dangerous narrative we've gotten ourselves into. That's why I game, for these moments when it is all on the line, and even I, the GM, have no idea how it ends. I got here by rules, prep, fiction. I can feel it when I don't prep. I find myself spending more time on 'downtime' and mundane encounters or scenes, avoiding moving the ball forward —so I don't have to be "on the fly." I feel like I'm cheating you when I do that rather than prepping for the session.
YMMV.