Aside from being a curriculum standard, I think it is important that students see the operations they learn about in different formats. It is also important that they apply their knowledge in different ways. I want students to understand the patterns in addition and subtraction, to notice that you start with a whole, and why and how it is different from addition. Fact families give a good visual to this concept. When adding three numbers, they can see that this is the same strategy as adding two. Until this is made explicit, students may assume they do not have the tools to solve these problems, when in fact they do. Later on they will learn about solving equations with missing addends. Having a strong foundation in fact families is important because it will give them the clarity and confidence to problem-solve these more difficult equations.