Python web application with C++ back-end

What does this statement mean exactly?

The best approach is often to write only the performance-critical parts of the application in C++ or Java, and use Python for all higher-level control and customization.

In the context of web applications and Saas, I understand that Python can be used for the UI and light weight data processing on the server. But when would I need to use C++ for performace-critical parts? I'm trying to understand how Python can be used with C++ to provide Saas or another web application.