Interaction design, like user experience design, has evolved to facilitate interactions between people and their environment. Establishing and maintaining a relationship with your customers is important if you want your business to be successful. Unlike user experience design, which accounts for all user-facing aspects of a software or system, interaction designers are (typically) only concerned with the interaction between users and computers. You could design and build the greatest looking website of all time, but your users could abandon the site after 5 seconds because they cannot interact with the website.
By creating the opportunity for interaction, you are creating an opportunity for building a better business. When a customer feels comfortable interacting with your business, they will be comfortable giving you business. Having that loyalty means you have built a successful website.