The .tar extension (or .tar.gz, ie. .tgz) in your files, comes from the "tar" command line that stands for "tape archive". 

tar was designed to read and write files to a serial device (magnetic tapes) without a filesystem. Being able to append files to an existing "tarball" was extremely handy (remember: slow, serial devices). Being able to extract only some files, too.

tar still works much the same, but most probably you use a filesystem file or stdio instead of a raw serial device.

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Dang, I didn’t know that. I kinda want to get an old magnetic tape storage system just for the fun of it.