Advantages

Compared to the default Git completion provided by Zsh, git-completion provides:

Much faster completion

When there are thousands of refs (like in the Linux kernel), the Zsh completion takes multiple seconds, while git-completion is instantaneous.

More up-to-date completion

Since the completion scripts come directly from Git developers, they are always up-to-date.

Better features

The features supported are the ones Git developers value most.

Proper switch/checkout completion

Depending on the --guess and --track options.

send-email

The completion for git send-email has been much more tuned by Git developers.

Automatic complex aliases

The Git completion knows how to figure out the right completion for complex aliases.

-C

git -C other-repo show <tab> lists the correct branches (from "other-repo").

All tests pass

All the completion tests from Git’s testing framework pass.