Comment by diggernet

A tar of gzip or zip files doesn't make sense. But gzipping or zipping a tar does.

Gzip only compresses a single file, so .tar.gz lets you bundle multiple files. You can do the same thing with zip, of course, but...

Zip compresses individual files separately in the container, ignoring redundancies between files. But .tar.gz (and .tar.zip, though I've rarely seen that combination) bundles the files together and then compresses them, so can get better compression than .zip alone.