I've said plenty of times in the past that the tests you write become excellent documentation which can be used to demonstrate how different aspects of your system should be used. This is very useful for someone joining a project. If it is well-documented in its tests, learning how to work with the code is as easy as reading through the tests and seeing how the different objects are intended to interact. If you can't do this, you're probably not testing well.

A good measure of your application's testing is to have someone learn to use the system based solely on the tests. If the tests are written well enough, someone will be able to figure out how everything works and interacts.

This is one of my primary goals when testing an application. I want to make sure that things are clear, because when I come back to this code a month from now I will be that new developer on the project. I will need to know how to use the classes, interfaces, etc. and having it documented in working cases goes a long way.