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.

zip doesn't retain file ownership or permissions.

The zip directory itself is uncompressed, and if you have lots of small files with similar names, zipping the zip makes a huge difference. IIRC in the HVSC (C64 SID music archive), the outer zip used to save another 30%.

Plain old zip is tricky to parse correctly. If you search for them, you can probably find about a dozen rants about all the problems of working with ZIP files.

It’s no longer needed. You can leave it out and it auto-detects the file format.