Sometimes you just want to add piece of HTML to your page that will always be the same - like a decorative line. Since you're not adding any data, there is no content-type to choose from. This is where the Design-Element is used.
Examples for this Feature
- Decorative separation lines (HR-tag with CSS or images)
- Recurring decorative element
- An invisible element, which may have an effect on print-page-breaks or similar
Special Use Case for the Content Area
This simple concept is as follows
- If you create a template without giving it a content-type, it will have "no content-type"
- When a user adds a "Content" module to a page he is offered with a choice of content-types - usually "Simple Content", "Person", etc.
- If there are templates without content-types, the dropdown will also provide a option called "Design Elements"
That's it :)