If you are flying in mountainous terrain, you will not know all details......
You will be able to calculate your weight at the time of arrival, but that's about it.
Wind situation up there is unpredictable, and so is the temperature.
Same with unknown LZ in the rain forest.... You don't even know the exact location-sometimes you just have a coordinate, and you know the recce team will be on the ground within a few km radius of that coordinate.
Now tell me how you are doing your calculations before the flight?
Especially in remote areas, with no weather forecast or reporting station within 1000 nm?
That's why Offshore pilots are usually unprepared when coming into the Utility world, as they are used to much better service around them... (let's not get into a debate about it again, Utility pilots getting into the Offshore world also struggle...)..