When changing lenses on the X5S, I can feel and hear the locking pin snap into the lens to lock it from turning.  Can't remeove it unless also pressing the release button.

So why the additional locking ring behind the bayonet?  Does this pull the lens in tighter for infinity or something?  Seems like overkill as the locking pin does work, and I rarely use the locking ring too (Didn't even know it was there for a long time too!).

I believe it is a seal to keep dust out.

This ring is used to erase the gap or virtual spaces between Lens and mount, make it stable and keep the dust and water vapor out.