Every once in a while I am surprised by somebody who says that prototyping is such a waste of time. For quite some time I have taken for granted that prototyping is a fact of my software engineering life. Maybe it is worth asking: what is the problem for which prototyping is the answer? Or, in other words: why should you prototype?
You should prototype when you don't know enough about about the problem. Which is almost always. When your knowledge is lacking in some aspect, then it is time to write a prototype. When you don't know enough about an API, some requirement, the operating system that you are designing for, the libraries that you want to use, and so on. When you feel that you have to answer a few clarifying questions, then it is time to answer them with a prototype.
The short answer is that you prototype because you don't know enough to proceed with the project at an acceptable level of risk.