The metal ring is used to remove the pen tips (nibs) from the pen. There seems to be no set line of thought about when you change out the nib. I've had two Toshibas (3505 and M200) and I've changed three nibs. Two on the 3505 and one on the M200. I did the latter just to see if it made a difference. It was neglible.
Here's another thread on the same topic.
Has anyone come up with a source for replacemet nibs?