Rubin presents a list of five reasons (pp. 4-8; pp. 6-12 new edition). Here is a brief summary of his reasons.
These principles help designers and developers overcome the obstacles for creating usable products and systems.
Many lists exist. Here is Rubin's list.
There are many different ways to describe and categorize evaluation methods. Here is one set of categories that Nielsen and Mack describe. They use it to motivate usability inspection methods.
Also see Lewis and Rieman's online text for additional descriptions of these methods.
In class, we will apply these methods to a simple unit conversion application (as time permits).