Using the STL documentation
The STL is a generic library: almost every class and function is a template. Accordingly, one of the most important purposes of the STL documentation is to provide a clear description of which types may be used to instantiate those templates. As described in the Introduction, a concept is a generic set of requirements that a type must satisfy: a type is said to be a model of a concept if it satisfies all of that concept's requirements.
Concepts are used very heavily in the STL documentation, both because they directly express type requirements, and because they are a tool for organizing types conceptually. (For example, the fact that ostream_iterator and insert_iterator are both models of Output Iterator is an important statement about what those two classes have in common.) Concepts are used for the documentation of both types and functions.