what is ,*, ?

[nilamo]

It's used in some internal functions, like list.sort().  The idea behind having arguments which are keyword only is to make code clearer. 

For example, if you pass a function to list.sort, does that function take two values and return which should be sorted higher?  You'd have to read the code and try to infer what was happening, or look at the docs for list.sort() and see what it says.  By simply forcing keyword arguments, you'd immediately see that code written like list.sort(key=some_function) is using a function to extract the value which is to be sorted for each item in the list, instead of actually doing the sorting itself.

The only real use case is code clarity.  It doesn't help you do anything you couldn't already do.  Use it only to help reduce ambiguity.