The biggest hurdle getting visitors to buy is trust - they may be hesitant to buy since they haven't used you before. So this sidebox pulls in the elements to help create that trust: a clickable authorize.net if you are using them, credit cards icons, and the ability to add in whatever else you might need such as a secure certificate symbol in admin define pages.
This sidebox has several easy set up features and the package also includes real credit card icons which Zen Cart does not include in the default installation and you don't have to create the HTML to display them!
See this blog post for why this is important: http://www.teamwiztech.com/blog/whats-wrong-with-zen-cart-checkout/7/
Available also in the template package: Zen Cart Ready to Go and demoed on this site on the right and with all credit card icons showing here.