The problems we're solving when building software happen simultaneously at different levels. Some are best solved by prototyping. Others are best solved on a whiteboard. The trick is correctly identifying the problems, and then knowing which layer they belong to. If you try to code up a prototype for a design problem, you'll most likely waste a lot of time and not reach any useful conclusions in the end (or end up shipping the broken thing). If you try to whiteboard a coding problem, you can get stuck forever drawing things, with no result to show for it.

In my experience, the problem levels go differently than one could naively expect. Data structures, abstractions, module interfaces - all problems dealing directly with code - are best solved first on a whiteboard, where evaluating and iterating through them is cheap and effective. User interfaces, user experience, usefulness of a part of a program - things dealing with business and user needs - are best solved through prototypes, because you can't reasonably think through them on paper, you have to have a working thing to play with.