The informal aspects of an interface, such as a high-level description of what each method does or the meaning of its result, can only be described in comments. There are many other examples of things that can’t be described in the code, such as the rationale for a particular design decision, or the conditions under which it makes sense to call a particular method.
These are great points; an additional point would be that writing comments makes one think more deeply about the philosophy of the method and what it is trying to achieve.
