It's useful to everyone who wants to avoid the stain on the internet that is Google Chrome.
Tempo is barely usable without the plugin, as it's so easy to lose track of where you've created timers across a large organisation. And half the time the app doesn't actually load on an issue when you click "open tempo" and requires a full refresh of the page to get it working again.
The Chrome plugin makes the experience of your software so much better, it's just a shame you have to extend the walled garden created by Google.