Thanks for that context. Why would I want to use it instead of git?

Because it's nicer tool to use. To borrow someone else's descriptions (paraphrased), it's simultaneously simpler than git and more powerful than git. Some things that were a source of friction in git (e.g. any non trivial rebase) just stop being so.

Easier CLI, deferred conflict resolution, megamerge workflow, an "undo" command which is like a gift from the gods.

Here's what I wrote about it eight days ago: https://news.ycombinator.com/item?id=45673808

Lots of other opinions in that thread too.

Because you work for Google, or because you don't want to wait for official git to be rewritten in Rust.

As I recall, git5 was such a piece of junk that they deleted it so nobody could use it any more. It had its fans but they were all deluded. It was a demonstrated fact that git5 users were less productive than normal g4 users.

What is/was "git5"? Git the vcs is only about to release version 3.0... Googling only reveals some gene that happens to be named that...

it’s an old google internal interface over google’s vcs piper that let you use git to manage your changes

It was a way to pretend that you were using git when you were in fact using piper.

The big problem with it was that you had to specify upfront which directories you will be editing, and only those were cloned locally. Later if you wanted to add a quick debug printout somewhere else, you would spend 20 minutes tracking more directories and merging / rebasing branches. All because git choked on the huge monorepo.

auto rebase, simple undo (jj undo), no staging area, easy splitting and merging