TAR makes multiple things into one thing, so that repetitions in those things will cause compression gains. Not sure how the other compression algos work, but a

TAR makes multiple things into one thing, so that repetitions in those things will cause compression gains. Not sure how the other compression algos work, but at least when gzipping, you will not gain that compression, if the repetitions are in separate files. So first TAR then gzip has been the standard way of doing it.