The reason that you would want to produce a high fidelity prototype is Engagement. No matter who are your stakeholders – executives, end users, developers, – higher fidelity prototypes attract and hold your attention much more than low fidelity prototypes. This engagement can lead to the discovery of more complex and subtle issues and conflicts during the design process.
High fidelity prototypes take time and cost more money, some say it's worth it.
When we are very early in the design process and are iterating with clients on general functional and/or navigation issues we use low fidelity or “Sketchy” wireframes (see http://www.boxesandarrows.com/view/sketchy-wireframes ) and “filler” text such as Lorem Ipsum. By combining what looks like a hand-drawn sketch with what is obviously filler text, we want to minimize the amount of time that clients comment on logos, color, fonts and content and maximize their attention on the information architecture.
A high-fidelity prototype is a prototype that is quite close to the final product, with lots of detail and functionality. From a user testing point of view, a high-fidelity prototype is close enough to a final product to be able to examine usability questions in detail and make strong conclusions about how behavior will relate to use of the final product.