I've been seeing some examples of Python being used with c++, and I'm trying to understand why would someone want to do it. What are the benefits of calling C++ code from an external language such as Python?
I'd appreciate a simple example - Boost::Python will do
Performance :

From my limited experience, Python is about 10 times slower than using C.
Using Psyco will dramatically improve it, but still about 5 times slower than C.
BUT, calling c module from python is only a little faster than Psyco.

When you have some libraries in C.
For example, I am working heavily on SIP.  It's a very complicated protocol stacks and there is no complete Python implementation.  So my only choice is calling SIP libraries written in C.

There are also this kind of cases, like video/audio decoding.
Practical point of view: Why would I want to use Python with C++?