Traditionally, unless you wanted the cost of keeping a couple of extra servers sitting idle in another data center, you would have back ups provided by your hosting company in their data center. Even when that was done right, which was only half of the time, there was a huge problem. Consider, for example, major data centers in Texas.
Just looking at major data centers in Texas alone, in 2008 The Planet data center had a fire which took out 9,000 servers, including EV1 and RackShack customers. If their backups or fail over were in the same datacenter, they were melted in the same fire. The same year, Alphared locked the doors of their two data centers, leaving customers with servers inside hanging.
The next year, in 2009, one of Core IP's former customers had hosted a copy The Wolverine movie, so the FBI seized everything in the data center. Everyone hosted at that data center was out of business unless they had a duplicate copy in another data center, like Clonebox.
The next year, 2010. Data center operator Acronoc shut down with no notice to customers when they are unable to pay their bills after unexpected AC repairs were needed. Customers are locked out of the DC for a week before being able to retrieve their equipment.
Every year another significant data center goes off line, either permanently or for a week or more. If you are in it the long term, with a business you aren't willing to give up in a Planet type fire, you must be able to fail over to, or at least restore from, a different data center. It won't do to have your back ups and failovers in the DC that the government seizes, not for business that intends to be around long term.
Does that mean you have to roughly double your costs by running duplicate servers at two data centers? No, not any more. By leveraging the newest technology along with concepts we've developed over the last decade, you can have these resources available only for when you need them on a "time share" type basis, so it's not a budget buster. It's only the storage that you are always using, for the four images of each server, while the servers themselves you don't pay for until you need them, using cloud technology.