Why we need cardPanel in Workspace?

Moving from document by Michael Tan

Recently I do the research for the Spark source code. And I find an interesting variable in Workspace class: cardPanel. cardPanel is an instance of JPanel, and Workspace itself also extends JPanel. Actually MainWindow should include three panels: statusBox, commandPanel and workspace. And we can find this line code:

cardPanel.add(WORKSPACE_PANE, this);

This means the cardPanel just adds Workspace as its component, in fact Workspace is cardPanel’s only one component.

So why we need cardPanel? I can’t find the use of it. Because we can use workspace instead. Could you tell me why we need cardPanel? Thanks!