So bassically you pre-script the scene, and let the players think they have some influence in the result, while it's actually a fate thing?
No. Completely the opposite. I presuppose very little, let the scene play out, and make judgments when they need to be made. I generally have no idea what will happen over the course of a session. I'm an improv DM.
And, as in any game, since I am the arbiter of the world (or the DM is, in general terms), the players' influence comes through me. If they build characters I like, make good decisions, and roll well, then good things often (but not always) happen for them (in D&D anyway; in CoC not so much). If they don't, the results are less inspiring. But as in any D&D game (or as in life, or as in all but the most extreme examples in fiction) one character has very little influence over the world and even over his own life. I mean, no matter what the players do, there's nothing stopping me from chucking the Tarrasque at them or making them deities or bringing about the end of the world. The DCs and modifiers for any d20 roll are whatever I say they are. It would be absurd to suggest that anything in the D&D rules gives the players any real power relative to the DM.
So why would I let my players think they have any more influence than they do? The influence they do have is more than enough to satisfy them. Trying to get them to assume greater narrative control is not necessarily a meaningful goal. The players want to be (in dramatic terms) actors, not writers, producers, or directors. DMs take those responsibilities. Playing one character and reacting to the DM's choices is a perfectly reasonable way to play.
I mean: the PC roll and win Initiative, but you ignore that, because the bd guys have some thing they are going to do that is already scripted. They do it, a PC cast dispel-magic or whatever, and undo the action, and you ignore it too, because it doesn't fit the story you are trying to tell. Later, and lucky crit would kill the BBEG, but you fudge his HP, because it is not "the right time to die yet". Later, once the NPC has done all his script things, when it's the "right moment to die", if the player attack miss, you fudge it too, and then he dies.
Since I don't have those kinds of things scripted very often, generally there is no need to do those things. If they get lucky and kill an enemy that was supposed to last, then that's the story of that session. If they get unlucky and die, that's the story. Part of being a DM in my opinion is adopting a certain equanimity; seeing the perspectives of the NPCs and the drama of the story but not becoming invested in any one character or plotline.
However, on the rare occasion that I actually do feel the need to "cheat" and modify or ignore the results of initiative, saves, damage, or other game rules to change the outcome (as opposed to just skipping the rules and getting to an outcome that was going to happen anyway, as described above), the results are almost invariably enjoyable, and no one complains. And I don't particularly feel I've overstepped my bounds, merely exercised my discretion.
Is it that? Why do you need rules then?
The main reason is to establish a framework of reality. Without the rules, every player would simply say that they attack and kill the enemy. The numbers on character sheets give us a way of describing how powerful each character is, and let us make judgments about relative power. The DCs for skill checks tell us what a character of a certain power level can realistically accomplish in the world. The dice allow us to interject an element of unpredictability.
And, of course, because building characters is fun.