As any expert IT specialist will tell you, electronic data degrades with time. For example, the computer disks (remember those?) and CDs from 10 years ago may or may not be recoverable when you dig them out and put them in your new computer. And stuff from 20 years ago you can basically forget about recovering.
However, with legal compliance requiring documents to be recoverable even decades later, a large company cannot afford to not have paper backups of their important documents.
That's where Iron Mountain comes in. They archive and protect corporate documents in an underground storage complex in Pennsylvania under tight security. This offers companies the benefit of not having to worry about their important documents, so they can get on with business.
From Iron Mountain's point of view, it's a great business model because the revenues they make are recurring, like a utility company, and there's a big switching cost for companies that want to take all their documents back from Iron Mountain and store them in house. It's easier to simply let Iron Mountain store their documents in exchange for the relatively small fee.