I use Hetzner and Bunny too. What does Coolify do in this instance?

Github-to-production or similar autodeploy workflows, cert management, backups, monitoring, notifications, and other things.

Coolify's website [1] explains it better.

But for example, you can call a webhook once new resources have been created, so Coolify can push them to your CDN.

[1] https://coolify.io/

It's basically an app store and service manager for self hosting. You can deploy databases, auth providers, Git forges, a static site, anything you want with one click in a nice web UI. Hetzner only gives you a base Linux install and everyone knows setting up Docker from scratch is a pain.