You use it to write Python code that runs on the .NET platform. It's good for anything that Python is good for.
The Python language is the Python language, no matter what. IronPython is an implementation of the runtime for that language - the support code that compiles your source, creates a virtual machine to execute the bytecode in the resulting .pyc file, communicates with the OS in order to put command line arguments into sys.argv, and all that other fun stuff. (Well, OK, the .NET platform itself does a lot of the work, too. :) )
