The last point is exceedingly useful. If you build lots of in-house packages you may have forgotten what depends on what and when you go to say 'clean room' upg

The last point is exceedingly useful. If you build lots of in-house packages you may have forgotten what depends on what and when you go to say 'clean room' upgrade to a newer version of X, Y and Z don't work or X must be built first. Of course these simpler things are handled by proper dependencies but if you always build on a box designated "we built packages here" those lines can blur. Doing from scratch reproducable build enforces getting those things right from the start.