As far as I remember, it was used to pass API requests for third party services (like APNS) through the main Bitwarden servers so Bitwarden's secret keys for these services weren't published, but self-hosters wouldn't have to register and manage their own accounts for these services, which can be complicated and expensive (To get access to APNS you have to pay the $100/yr apple developer subscription and you can only use it for your own apps, so you would have to build and distribute, via the app store or testflight, your own build of the app.