DITA is a topic-based architecture. It allows you to order, reorder and nest topics to create any kind of information product.
Topics can be used in more than one deliverable at once, independently from other topics.
Content created according to DITA-principles can be transformed by any DITA-aware transform even if the creators are using different tags derived from the DITA topic.
Because DITA's architecture is based on topic structure instead of a specific output structure, like a book or an online help system based on stand alone topics it allows for topic re-use in multiple output format and stays coherent to users' context. Topics can be added to and deleted from each specific output, without affecting other outputs. It's like building multiple pieces with the same available Lego blocks. Topics can be presented one at the time in a simple flat online help system, or together in a complex multi-level book structure.
Because DITA is more of an architectural base than a fixed DTD/schema, you can adapt the tag vocabulary to fit your own specific needs.
Instead of starting over every time you're in need of new DTD/schema elements, you can specialize existing topics to fit your precise projects' needs. New topic types can be created from existing ones, which allows for evolution over time. One of DITA's main advantages is that adding new topics does not make previous projects incompatible with new ones. In fact, DITA allows for children elements to inherit their parent's transformation.
You can inherit elements or create new ones based on existing elements. Inheriting common elements saves time in terms of tag definition and transformation development. You never have to start from scratch.
Because some DTD elements are derived from others, they can inherit their properties. Style sheet formatting rules apply automatically to derived elements. Yet, you can modify that behavior, if need be.
Topic reuse means you do not need retype or copy and paste information in multiple outputs. It also leads to a need for fewer modifications as the product and its documentation evolve overtime.
Topics used in many deliverables must only be translated once. Moreover, as the products and their documentation evolve, only modified topics need to be translated.
DITA is modular and extensible; therefore it does not pretend to cover everything.
DITA DTDs/schemas can be used as is. All most frequently used elements and topic specializations are available. Moreover, generic elements like paragraphs can be used in multiple contexts.