It differs from other such applications by the fact that it uses the parse tree for a module to derive the information used in its output, rather that importing the module directly. This allows the user to generate documentation for modules which need special context to be imported. This is an interesting concept, there must be a ton of useful information embedded in the source code of software projects. Code analysis is by no means a new field, but the creation of documentation from code analysis could be an area that is worthy of further study. Happydoc works on this principal, and there are, no doubt, many other examples of this kind of code.
Comments can also be out of date or misleading.
Nonetheless, looking at all of the comments in a body of code as a whole could be a useful method for deriving some useful documentation from code. Trouble areas in the code would likely stand out. In fact, the comments in the Linux kernel source code were analyzed and a lot of colorful language was revealed. Through the use of Perl, Python, or other high level languages, it should be possible to write a program that looks at a piece of code from all of these angles and produces a pile of output, some of which could be useful for writing documentation. Documentation is famous for being left as the last task, or never finished.