I won't deny I'm also asking this because I do not personally see a reason... after all, I wouldn't ask if it were obvious to me. But I really don't understand what this is good for; it seems like binding Erlang to a Python library bound to underlying C and assembler code is doing nothing but bringing you disadvantage to accessing the Python with no corresponding advantage I can see; for instance, it isn't obvious to me that you can get any utility from Erlang's multiprocessor capabilities. It seems like, even on Erlang's terms, you'd be better off writing a Python program that does your task with numpy or scipy and having Erlang pipe whatever data it may be in possession of to a Python process in some more conventional fashion.
But I'm interested in whatever answer there may be. (Seriously.)