In his context, it really has to do with references. So, if I have a value A, and it's a reference type(either by pointer, or by just the nature of the languag

In his context, it really has to do with references. So, if I have a value A, and it's a reference type(either by pointer, or by just the nature of the language), then if I pass that object into function DoSomething, DoSomething may change A without my knowledge, and cause behaviors further on that I wasn't expecting. If I inline DoSomething, I see exactly what I'm doing to A, what I'm changing on it.