I'm happy to begin and end with the fiction, but if as a player the game lets me try to establish some of that fiction (that's not yet been established) via action declaration incuding intent, and on a successful roll that fiction becomes established, then why wouldn't I always declare intents that benefit my character and or its goals?
Because the game isn't about playing on God Mode, which is neither fair nor fun.
And also, because you don't get to declare just
anything. You usually (IME) a list of things to choose from a small number of options. A move to rob a place might say that on a success, you choose one (partial success) or three (full success) of the following: you don't trip any alarms, you leave no evidence behind, there are no witnesses, you get what you came in for. So if you had come in for the Desert Rose, you get that--but on a full success, there's still a bit of a problem. It may not be a
major problem--maybe you tripped alarms so they know the Rose was stolen sooner than you would have liked--but it's still a problem.
If they would have died anyway had I done nothing, my failing to get into the building didn't change anything.
OK, change of tack.
You are in combat. You're down to your last few hit points--one more blow
will kill you--but you
know your opponent will die if you hit them one more time as well (maybe you have a trick up your sleeve; maybe you've been counting their hit points and know how many they started with). And your opponent is powerful enough that they will almost certainly hit you, even on a bad roll.
You have the initiative.
You roll to attack. You miss.
Because you missed your attack, you didn't kill your opponent, and therefore, you will almost certainly die when they hit you.