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.
Here's two possibilities:

1. Perhaps the C++ code is already written & available for use.
2. It's likely the C++ code is faster/smaller than equivalent Python