Apple also uses zip for Keynote, Pages and Numbers documents, again for very much the same reasons, and Java's .jar, .war, .ear, .whatevar files also use zip.
It also, unlike tar.gz, provides an index for easily seeking to and extracting just one file from the zip archive. My understanding is that doing this in a .tar.gz means unzipping and processing the whole tarball as a stream.
.zip seems to be the universal 'combine some stuff in a single file' format, probably because it is old enough to be guaranteed patent free (OTOH: http://broadcast.oreilly.com/2010/06/is-zip-in-the-public-do... and http://www.pkware.com/support/zip-app-note. http://www.pkware.com/documents/casestudies/APPNOTE.TXT only mentions encryption and patching as patented)