Credit for the complete explanation/answer to this question was posted back in 2012 by @Thilo in the context of a different question:

That "object" parameter to "addObserver" is an optional filter. Upon posting a notification you can set an object to the sender of the notification, and will then only be notified of that sender's events. If set to "nil" you will get all notification of this type (regardless who sent them).

My learning continues.