Good question. There's a few key benefits:

- It builds on the existing site, so the new design is always applied to HN urls. No need to visit a separate website.

- Privacy - everything is stored locally in your browser, there's no 3rd party website to track your activity etc. Also with JSON backup you're in full control of your data.

- Unlimited storage - local browser storage has limits, but extensions can request unlimited storage. This is useful for the history and bookmark features.