Imagine a library that internally uses GStreamer's playbin to provide certain functionality, like libwebkitgtk perhaps. Imagine that library needs to make playbin pick a custom http source. It will have to register a custom source that has a higher rank than e.g. souphttpsrc.

Now imagine the application also wants to do something with playbin. Now the application playbin will inadvertently also use the webkit http source, which may or may not work.

One could solve this by either providing modifier ranks on a per-pipeline basis, or adding API to playbin/uridecodebin etc. that lets users override the source selection.