dynamic_sidebar() callback is useful for, if you have no sidebar.php file to query and; for using sidebars that may not have a template. Hence the term “dynamic.” By dynamic this means the sidebar may have been generated in the themes register_sidebar(array($arguments)) function.
As @Codex demonstrates, it is always good to verify the sidebar is ACTIVE (set in the Widgets admin page) and not empty. This prevents an empty div section on the page which might create empty space or broken alignment of page view. Using PHP conditional statements like else, if or case switch; in your template is the safest way to render a dynamic sidebar. It also give your theme context if the admin has not found the time to put a sidebar into a widget.