Earlier quoted context omitted.
Because you might need it to share documentation with customers. Confluence isn't just for external documentation. Confluence, at it's core, is just a wiki. Sometimes it needs to be available online, sometimes it really doesn't.
If you’re ok sharing things externally why self-host at all?
There's a fair few reasons other than this, it's not unthinkable to host servers yourself if you already have other servers on-prem anyway.