Prototyping isn’t just the job of your designer. It’s more about the collaborate process involved in figuring out the specifics on what to build and how to build it. It’s also highly situation dependent. In general product management prototyping is about validating ideas, helping explain those ideas to others and then iterating on said ideas. With today’s rapid prototyping tools, it’s much faster to build a basic prototype than to build a fully fledged version of the feature, only to discover flaws when it’s in being used by an actual customer.