Why have SSNs? Easy
You need to provide your social security number when you apply for credit. Why? I don't know. Everybody seems to have decided that it's a universal identifier. Which is why I watch mine like a hawk. A few years back we got a Best Buy card when we bought our TV. ($100 off to buy it with their card). When the guy was finished calling in the information, he dropped the application in a shredder. THAT's what Radio Shed should have done. The credit application probably had name & address, SSN, date of birth, home phone number, employer and, quite possibly, mother's maiden name. All the makings of an ID theft.
As a security consultant, I've done quite a bit of dumpster diving. You'd be shocked at what some businesses put in their trash. I once found a copy of a marketing plan marked "confidential" that was dated three days earlier.