silverstrata
I'd say because their procedures require them to ask and while it's on a load sheet somewhere retrieving that load sheet for a flight originating in a different country may take some time (or will have taken some time when these procedures were devised in the age of telex). And there are lots of GA planes around that don't have a load sheet filed somewhere.
As for the fuel it might be useful for the fire department to know whether to prepare equipment for a fully loaded 777 or a Dash 8 fuelled for 90 minutes.