It’s a simple question, for explanation. It’s not meant to antagonize anyone.
It is a simple question, but let's be clear - first you asked:
"If a DM fudges die rolls to make the experience best, why roll the dice at all?"
Then you asked:
"In situations where the DM will only accept a certain result, (will fudge a roll to get the result he/she wants), why roll at all?"
Surely you see those are different questions, no? The first is kind of absurd. The second is better, but shows a lack of understanding of what the GM is probably doing.
When I fudge a die roll, it is not because I have a clearly preconceived and detailed notion of exactly how things should turn out. It is that I see the immediate result of one die roll as being particularly undesirable. I am not forcing all dice towards one conclusion, I am forcing one die away from one conclusion. There's still a world full of other results that I'm not going to alter.
Why did I let you roll the die at all? Well, there are several possible reasons. First off, I may not have foreseen that conclusion - I'm not perfect, after all. Second, there's the concept of "if it ain't broke, don't fix it". I'll let the dice speak until they start using foul language, and then only bleep the really bad bits. That doesn't mean I've written their script in full, you see.
So, the BBEG rolls an attack, and hits. Well, that's okay. Rolls a crit. That's still okay. Rolls max damage such that he'll drop the PC in one stroke before that PC has even gotten to act in this climactic battle, so the player's going to be sitting out for the next half hour of the session feeling useless? Maybe not. Maybe I'll tone down that damage.
That doesn't mean I wont let the PC die. Just not on that particular hit.