It's about manageability, disaster recovery, and the strategy of backing up remote branch offices. For instance, I may have a DC in Building 0 in Seattle, but I'll have 5 branches in 1, 2, 3, 4, and 5. What's the best and cheapest way to back up this data? Then, how do I track these backups from a central management perspective? Also, bandwidth costs can be insane.
So OST gives you a centrally managed approach. It allows you to back up across many types of storage technologies, it allows you to reduce the cost of disk capacity and WAN bandwidth (with "WAN optimized image replication to DR sites") and overall reduces the load on your servers. It will initiate all the data movement (images are created, duplicated, deleted, how to control images that are stored and copied).
Using your typical "backup to disk" doesn't address the complications of remote branches, per se. Or I should say, doesn't address the BAD associated with backing up from remote branches. It's expensive in a variety of ways, and OpenStorage was meant to address that.
I'm also going to include a ESG Review on the product that will give you a bit of more in depth view /lab setting: link.