Is content reuse important?
How much of your content gets reused in other contexts?
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. This requires content that is topic-based and written for reuse by avoiding explicit references to context.
Content reuse cost savings increase greatly when your content goes through a workflow with distinct review and approval stages, for example legal approval. Content that is reused can generally avoid all or most of the extra steps in the workflow that involve accuracy of content. You will still need design approval of the in-context appearance of the reused content.
Reuse in DITA can be reuse of a whole topic, for example in a new ditamap that uses a different set of topic references (topicref). Or it can be reuse at the subtopic level, where a part of a topic is given a unique ID and then included as a content reference (conref) in another topic.
Partial reuse is when much of your content remains the same in a product life cycle change. In these days of shorter and shorter product release cycles, partial reuse can save not only money but the time needed for production of the revised materials. And time is money.