When we join a new team or take on a new project, we are usually inheriting a large amount of code. Our ability to be productive in these inherited code bases is dependent on our understanding of them. So, before we even seek to make the first change, we need to build in our minds a conceptual model of how things work. It's not necessary for it to be exhaustive and complete, but it must enable us, at a very minimum, to make a change and understand exactly what effect that change may have on all the moving parts of the code base.