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: 
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
One nice thing about using a scripting language is that you can reload new code into the application without quitting the app, then making changes, recompile, and then relaunching the app. When people talk about quicker development times, some of that refers to this capability.
A downside of using a scripting languages is that their debuggers are usually not as fully featured as what you would have in C++. I haven't done any Python programming so I don't know what the features are of its debugger, if it has one at all.

This answer doesn't exactly answer what you asked but I thought it was relevant. The answer is more the pro/cons of using a scripting language. Please don't flame me. :)