You'd save in the old format if someone needed to work in the file who didn't have PowerPoint 2007 or the Compatibility Pack for opening PowerPoint 2007 files.
The Compatibility Checker, in the foregoing example, indicated that a new graphic would become one object, like a picture, and therefore uneditable, if the presentation were saved in the old format.
The person opening this file in PowerPoint 2003 would see the PowerPoint 2007 graphic with the truest visual fidelity possible. She would not be able to edit individual shapes in the graphic. She could add a backfill color behind the graphic, or apply other picture-type commands, and resize it. But she couldn't work with any of its parts to change its layout, text, shape style, or the like.
"Roundtripping" If the graphic is basically untouched by the person working in an older PowerPoint version, it will retain all its PowerPoint 2007 properties. So when it's opened again in PowerPoint 2007, it will be fully editable.
If the graphic did get some change when in the older version, such as a picture washout, it becomes one object, and thus uneditable, when opened again in PowerPoint 2007.