Some libraries still ask for social security numbers on their library applications. Others have stopped that practice, but haven't purged their patron record databases of these numbers.
Yes, collection agencies want the numbers, and perhaps having this information can increase your success rate in tracking down scofflaw patrons.
But consider the downside. If someone hacks your database, or if you have a bad employee, this highly sensitive information is at risk. Once it's gone, it's gone. Patrons have little recourse once identity thieves get their hands on these numbers.