The sentence below needs a comma because it is parenthesis in apposition.

"She was an interesting talker, a woman who had traveled all over the world and lived in half a dozen countries."

In other words, talker = woman.

The sentence below gets a comma under a spin-off of the non-restrictive ruling.

"I met them on a Cunard liner many years ago, coming home from Liverpool to New York."

The comma basically says "I am a clause that does not modify the thing to my immediate left."

These examples make it clearer:

I swallowed a fly riding my bike.
I swallowed a fly, riding my bike.

It's a really great question. This topic is not well covered in books.