web2py makes extensive use of these. Take the A, DIV, or TABLE as examples, you can pass HTML attributes (e.g. _class, _id, _style) to these, but if you look at the code, you will notice that these things are not in the signature of these methods. They allow you to provide an unknown number of parameters to a method. For example:
def my_method(text, _class='', _style='', _id='', _onclick='') Could easily be rewritten to: def my_method(text, **kwargs) Then kwargs is a dictionary with any extra parameters passed: if '_class' in kwargs: output += 'class = "%s"' % kwargs['_class'] Using *args on the other hand would be if you didn't expect named parameters to be used, just an unknown number of unnamed paramters. Hope that helps to clarify.
