One of the challenges in UI design, specifically interaction design and how it turns out to impact user experience, is properly managing UI artifacts (widgets) such that they are compact and pleasing on the eye but easy and handy to use. So how do you put together buttons that do related things? If you have similar things to create or related reports to show and you place them (as buttons) all on ButtonBar, you'll soon run out of space. Even if you have enough space, there'll be this monotony about their presentation that will make them boring.
This is the sort of thing that the Menu Button and Split Button are designed for, helping you present an array of related actions with a drop-down (well depending on the alignment) menu when the button is clicked.
