Good information architecture helps customers find what they want and keeps them coming back.
Your site's architecture is supposed to help visitors find what they are looking for. Categories and links should make sense to your customer so that they are able to find information on your website, whether it be products, contact information, articles, or whatever. Navigation should be natural: know what your customers will be looking for and provide links that obviously contain that information. Easy usability helps keep customers at your website.
Of course, there is more to information architecture than being able to find the way through a website. You want your customers to do something while they are on your site. People who click through your entire website but don't take action (i.e. purchase a product) isn't what you are looking for. Don't leave what your customers do up to chance; get them to take action. The biggest way to get customers to purchase is to persuade them to do so.
Combining good functionality and organization with persuasive copy is key to making your website work.
Putting all of these together helps customers stick around longer, come back more, and take actions you want them to.
