What is the purpose of these spare rings for an HVAC blower motor?
(Credit for this answer to user d.george, who has not responded since posting as a comment.  I'm posting as an answer so this question can be closed and marked answered.)

The rings are to make the motor mounts of a 48 frame motor adapt to a frame 56 base. If the old motor was a frame 48 and the replacement is the same, you do not need the rings. If the base was originally for a 56 frame motor then you slip these rings onto the ends of the motor before installing. (1 per end) If your motor does not fit into a base or if you don't them , then you discard them.