Question: 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

Answer: 
Generally, you'd call C++ from python in order to use an existing library or other functionality. Often someone else has written a set of functions that make your life easier, and calling compiled C code is easier than re-writing the library in python.

The other reason is for performance purposes. Often, specific functions of an otherwise completely scripted program are written in a pre-compiled language like C because they take a long time to run and can be more efficiently done in a lower-level language.

A third reason is for interfacing with devices. Python doesn't natively include a lot of code for dealing with sound cards, serial ports, and so on. If your device needs a device driver, python will talk to it via pre-compiled code you include in your app.
1. 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.

2. 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.