Too many times in the past have I been in a room with an internal team trying to figure out how to name and arrange menus on a website. It doesn't need to be that way. The principle is simple. Let a bunch of your users drag, drop and rename menu items and run statistics to find the best compromise. Card sorting can remove a lot of guesswork out of your information architecture and taxonomy efforts. They are important because internally we often categorize things differently than our users. Here is a public analysis I ran so you can better understand card sorting and what you can expect from Optimal workshop.