Summary Tasks
A special type of task is the Summary Task.
A Summary Task is useful for displaying the project the way upper management sees it, minus the details and answers the question "how long will the whole thing take?"
A Summary Task is a task that acts to show the total duration of a group of "sub- tasks".
It's displayed as a summary bar.
graphicSummary Bar

As in MindManager, where you can display many levels of branches, in the Gantt, you can have many levels of projects and subprojects.
graphicSummary task as shown in Tree Pane

Any "higher level" of tasks is shown as a summary task with a black, dull, color- less bar...perhaps it's a metaphor for "we like our information black and white"
What are they good for?
If you're running 5 different projects simultaneously and have to present your current status to the executive committee, you can collapse the branch so that sub-tasks are not shown, and simply show a graph with the Summary Tasks.
That way, they don't need to know the details as to what you expect will cause which action to be delayed, they can just see the overview of "what's scheduled" or... "what's occurred".
Tip: You can set the % done on a Summary Task as you see fit, according to your own metrics.