Baggage weight
 

From what I've seen, the main reason computer systems (DCS , baggage management ) are asked to provide actual bag weights is so that the cargo dept. can send over enough freight to fill the aircraft to capacity. Or, if its already at the stand, how much of it can actually get loaded.
Interestingly, this implies that using standard pax & bag wts may - on average - err on the high side...

Years ago when flying with ExCalibur on the A320 we were getting check gross weight messages on the FMGC and the fuel burn was a few % above the book.

We were using assumed weights for hold baggage so we decided to start weighing the hold baggage (and incidentally on flight to FNC we weighed cabin bags too). This showed we were often heavier than we had previously thought. We never got anymore FMGC messages and the fuel burns were almost exactly as advertised.

As and aside it seems quite amazing in this day and age of modern technology that we do not have an accurate system on the aircraft which tells us what the actual weight is and where the c.g. is located.