[dfox]

Apart from various technical reasons the workflow of essentially all linux package managers is built on the notion that the binary package is built by some well defined (and repeatable, at least in the "does same thing" sense) process from some kind of source package. Also the souce package format usually contains mechanism that allows distributions to comply with copy-left licenses while also cleanly documenting which modifications were made to the package from upstream version (it is sort of special purpose versioning system).