“Zif is a package management backend for PackageKit, designed to do things right. It preforms much better than yum with PackageKit UIs, making the UI output results much faster. As yum is more tested and used than zif, we should keep yum for CLI by default.”
(from the feature page).
With zif, PackageKit UIs (such as gnome-packagekit) are much faster and more responsive. For example, search result appears in less than half a second when using the zif backend, while it sometimes take more then a minute with the yum backend.
More information in the feature page.