What makes signet rings so important for noble and armigerous families?

Are they symbolic of some sort of succession? Given by the father to the oldest son as symbol of his trust and blessing to succeed to him? I know the history of signet rings and how they originate from Ancient Rome, but how about their meaning in the middle ages? How about their meaning now? Are they just an accessory? Do they still mean something in an age in which every common folksman can wear a signet ring even if he is not entitled to do so? Is it still used as a method of visual recognition of nobility or gentry?