Because you might want to display an up-to-date status permanently in a UI of some sort (e.g. an IDE or editor status bar or a shell prompt — the latter seems to be the use case here), and on big repositories it can be noticeable. Takes 150ms on the rust repository on my machine, with warm cache, that's a serious stutter when it runs on every shell prompt.

The tool's page specifically quotes Chromium which takes ~300ms to status on the author's system.