To allow users to evaluate proposals for the design of the eventual product by actually trying them out, rather than having to interpret and evaluate the design based on descriptions.

An HTML prototype of the site or application is made and tested on users once wireframes have had sign off. With immediate feedback, usability and interaction design issues can be resolved quickly before major development work begins.

It is best to use prototypes when they are working wireframes and not graphically rich or showing services from the backend. This ensures the user has focus only on the elements that are being tested, that of interaction and usability of the site. They should also have a minimal cost implication by being produced quickly.