Differences between MinimServer & BubbleUpNp server

[simbun]

BubbleUPnP Server is not a media server, it extends and adds compatibility to existing (UPnP and non-UPnP) renderers and UPnP servers.

The reason everything is still working in your environment without BubbleUPnP server, is because the 2 apps you're running have the ability to talk directly to chromecast, if you'd used a spec compliant UPnP control point the chromecasts wouldn't appear, unless you ran BubbleUPnP Server which allows you to turn the Chromecast into a DLNA/UPnP or OpenHome renderer.

Most people install it to add OpenHome functionality to their UPnP renderers so that the playlist queue is held "on renderer" (actually by BubbleUPnP Server), meaning your phone doesn't need to be orchestrating the queue between the server and renderer. Another asset is that it enables you to access your UPnP media server through the internet.