In “The promised land of prototyping: Realizing the full potential of prototyping”:http://www.guuui.com/issues/04_05.php Henrik Olsen provides 11 very good reasons as to why you should include prototyping as an intregal part of your web development methodology.
I particularly liked his comment about the benfit of providing the client something to look at:
bq.. *Can make the intangible tangible*
Clients have a very hard time understanding how a product is going to work from technical specifications, and developers often find it difficult to translate them into practice, because they usually describe what a product can do but not how the users are going to access its features.
Compared to technical specifications, a prototype is easy to grasp for everyone, since it’s a representation of the final product. It shows what the product can do and simulates how it’s going to work.
p. Amen to that.
Personally, I’m a big fan of “paper prototyping”:http://www.uie.com/articles/looking_back_on_paper_prototyping/ as I find it produces a large return for a relatively small amount of effort.