At the bottom of the window, you see the Status Bar. This area is used for two purposes: On the right, it shows you the total count of files and folders on your disk. On the left, it shows either what's being scanned (if something is being scanned by SpaceMonger), or whatever you have selected.
While the Status Bar is useful, sometimes you want to know more than just "3 files and 5 folders are selected." What if you're zoomed out and want to know more, or see where the files are in the treemap? Just knowing how many there are doesn't help. That's one of many times when the Selection Bar comes in handy.