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: 
Because C++ provides a direct way of calling OS services, and (if used in a careful way) can produce code that is more efficient in memory and time, whereas Python is a high-level language, and is less painful to use in those situations where utter efficiency isn't a concern and where you already have libraries giving you access to the services you need.

**If you're a C++ user**, you may wonder why this is necessary, but the expressiveness and safety of a high-level language has such a massive relative effect on your productivity, it has to be experienced to be understood or believed.

I can't speak for Python specifically, but I've heard people talk in terms of "tripling" their productivity by doing most of their development in it and using C++ only where shown to be necessary by profiling, or to create extra libraries.

**If you're a Python user**, you may not have encountered a situation where you need anything beyond the libraries already available, and you may not have a problem with the performance you get from pure Python (this is quite likely). In which case - lucky you! You can forget about all this.
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. :)