Do you publish content to multiple output formats and channels (e.g., web/HTML and print/PDF, perhaps online Help and someday mobile platforms)?

When you have one source for each piece of content, you get the astonishing ability to change it in one place and have the change propagate everywhere. A product name change becomes much more manageable. Write-once, read many.

Your business-critical marketing messages are standardized everywhere. Some call single source a "single source of truth" because you are assured that your customers are not getting mixed messages that can confuse them, reduce sales, and increase the need for tech support.

Some people define single-source publishing as providing multiple output formats like Web, Print, and Online Help.

Reusable content has a single source, of course, but reuse generally refers to content originally developed for one context that can be reused in another.

The ROI on single-source publishing appears to be an intangible when planning a new system. Hard cost savings are hard to calculate. But the ability to make rapid changes in critical content has obvious marketing and sales advantages.

Up to Making the Business Case for DITA.