The main benefit is the size of the saved page. The file will be smaller because binary resources (e.g. images) are not encoded in base 64 [1]. Moreover, the page and these resources are also compressed. The other benefit is that you can unzip the saved page and edit it more easily than a page saved with SingleFile because the saved page won't contain data URIs [2].

[1] https://en.wikipedia.org/wiki/Base64

[2] https://en.wikipedia.org/wiki/Data_URI_scheme

In fact the SingleFileZ format is quite magical : It's SingleFile that save the content of the page and the Z stand for Zip.

The SingleFile saved content is saved in Zip format in the result Html page.

The final SingleFileZ html page has Html header, Zip file in the body and a bunch of Javascript to decompress the content when the browser opens it.

It's magic !