The dual drives come with Dell DataSafe, which can be set to back up the user's Documents folder automatically. DataSafe also means that the extra hard drive is there as a mirror of the primary hard drive, so the user is protected from the failure of one of the drives. Ideally your workers will be storing important documents on your server, but most people like to work locally whenever possible. Strategies like DataSafe will save your workers' hide. It's only a matter of time before a hard drive failure, virus, or mistake results in lost files, which DataSafe can recover.
The 755 is useful for businesses that needs a little bit more than the standard desktop PC for their workers. Its dual hard drives give data redundancy to safeguard files. The OptiPlex 755 is a highly recommended business PC that also happens to be energy-efficient and environmentally attractive.