I want to mention here an important property of harmonic functions, the mean value property, and some of its consequences. I restrict myself to functions of two variables for clarity.
Many important properties of harmonic functions (and, by extension, of analytic functions) can be established solely in the basis of the mean value property. I don’t know how to prove this property (or that it characterizes harmonicity) without appealing to Stokes’s theorem, or one of its immediate consequences (the topic of Chapter 14 of the book); in fact, I doubt such an approach is possible. It is a good exercise to see, at least formally, how this result gives the mean value property, but a rigorous treatment tends to be somewhat involved. Unfortunately, the arguments that show that the statements below hold tend to require techniques that are beyond the scope of Calculus III, so I will skip them.