What is the difference between source and destination packages when transfering some products ?

Why the weight of the package(s) are not added to the weight of the products in the sotck.picking view (weight, weight net) ?

Edit:

Ok so I found this module which works around my second question:

stock_picking_package_info

Let's say we want to send a package to a customer. The destination package would be the package sent. The source package would be the package delivered to us by the supplier, if we didn't unpack it yet.

Am I right ?