Mega menus are an extension of conventional dijit menus that provide extra functionality such as:
Mega menus are commonly used as sophisticated navigation drop-down menus from a Header.
In particular, they can be used to provide a designated message area in a header. See: Messaging
They can also be used for pop-up, drop-down, and tooltip-style menus that are associated with arbitrary page elements.
Here's an example of a hover-triggered, multi-column mega menu with extra content working as a drop-down menu from a header: