Re: weight calculation
You do these calculations before you load the cargo into the plane. Having the plane measure it and then say "oops, that's outside the safe limits, you need to unpack and rearrange" is sort of not helpful...
It very much IS helpful for the pilot to know that the ground crew has loaded some heavy freight in the wrong hold, or failed to load such freight because it was delayed. Because it is far better to know that the W&B is out of limits when the aircraft can be unloaded and re-arranged than to only realise after take-off when the pilot does not have enough forward elevator authority and the aircraft stalls into a housing estate from 500 feet.
Or to get at least some advance warning that all the passengers are walking to the back because the steps for the front door have not arrived, and the aircraft is just about to tip onto its tail (has happened more than once).