This option allows you to have, for example, multiple files open at once, maybe with the windows tiled, etc. and save the 'workspace' exactly as that.  By taking this option, if you regularly work across two or three files, you will be able to open this one workspace file and all the documents 'contained' in it will be opened. The individual 'workbooks' inside a 'workspace', can still be opened individually in the normal manner. It presents a 'Save Workspace' dialog with Workspaces selected in the 'Save as type:' drop-down.