Some languages can use a word or phrase with a noun to give information about that noun or to remind people of something about it.
Mary gave some of the food to her sister, who was very thankful.
The phrase “who was very thankful” immediately follows the word “sister” and informs us about how Mary’s sister responded when Mary gave her the food. In this case it does not distinguish this sister from another sister that Mary might have.
Reason people use these phrases: People often present either reminders or new information in a weak way. They do this when they want their listener to give most of his attention to something else they are saying. In the example above, the speaker wants most attention to be given to what Mary did, NOT to how her sister responded.