They don t execute, and our customers pay us for working code, not pretty pictures. We bother with models to communicate. The idea is that a graphical object model can show how objects fit together more clearly than looking at the source, an interaction diagram can show a collaboration better than figuring out the call path from several class definitions. But so often the design documentation fails in this, and leaves me puzzled on my sofa. Part of the problem is the CASE tools that people use for this kind of work.
CASE tools encourage a dictionary mentality.
You make an entry for every class, you show every class and every attribute on the diagrams, you draw an interaction diagram for every use case. But that question is the wrong question. If you document everything, you are giving everything an equal weight. Do that for a complex system, and you are buried in detail. In any system there are some aspects that are more important than the others, key aspects of the system that once understood, will help someone to learn more.