How would "bit-identical" or "free of side effects" make an actual difference in practice? Rollback is already very easy with filesystem snapshots. Configs are

How would "bit-identical" or "free of side effects" make an actual difference in practice? Rollback is already very easy with filesystem snapshots. Configs are already tracked by etckeeper. New laptop: either copy the whole drive or the package list and dotfiles. Also, how often do you have to get new laptops for this to be relevant ?