Document indexing is the process by which key fields are entered for your documents to be retrieved by later. Organization is the key to finding documents quickly and reliably. Lets say you store your documents in folders in file cabinets with little tags sticking up on each folder which contain a social security number. The social security number would be a great example of an index field. Each scanned document would have a social security number associated with it for retrieval purposes. The more of these fields you have the more expensive the process becomes.
When you scan your documents, you eliminate the need for physical paper storage, which means no more file cabinets, file folders, filing processes, or the associated staff to manage it all. Scanning also allows you to reclaim valuable office space. Paper requires a lot of physical storage, mainly in file cabinets.