The separation of content from formatting is the key to obtaining the big three topic oriented writing advantages: 1) Single source, 2) Reuse and 3) Repurpose.
Single Source
The term single source relates to the concept of storing all content used by many users for many deliverable in one place. The advantage of single sourcing is that changes to single sourced content propagate to all deliverables that reference it when the deliverable is rebuilt.
Reuse
The term Reuse is defined by the practice of reusing content from any topic file over again in different sections of a mapped set of topic files, or for additional content referenced for reuse by another topic file.
Re-Purpose
The re-purpose of topic allows topic files from different or the same subject to be used in different sections of a mapped set of topic files.