You can use the principles to understand the practices better and to improvise complementary practices when you don't find one that suits your purpose. While the statement of the practices is intended to be clear and objective (for example "write a test before changing code"), understanding how to apply the practice in your context may not be obvious. The principles give you a better idea of what the practice is intended to accomplish. Also, no fixed list of situated, context-dependent practices covers all of software development. You will create new practices occasionally to fill your specific need. Understanding the principles gives you the opportunity to create practices that work in harmony with your existing practices and your overall goals.