As of version 1.60.19, BT747 has support for KMZ output in its desktop and command line version.

Why now? What are the benefits?

Basically, a KMZ is a compressed version of KML. Everybody can build a KMZ file from a KML file:
1) 'zip' the KML file into a Zip Archive, and,
2) change the extension from '.zip' into '.kmz'.

However, to do so in a program, one needs to implement or have the compression functionality available. In 'J2SE', compression functions for creating zip files are readily available. 'JAR' files are zip files themselves too, and this explains the availability, in SuperWaba, the code is stored in another format.
So as BT747 has its Desktop GUI implemented in J2SE, adding KMZ output was almost a piece of cake.

What are the benefits?

Due to the compression, KMZ files are about 10 times smaller than KML files. Because of their size, the will upload and download faster to the net. There are no other benefits that I can think of.