Workbook helps the user to better organize and to group different data containers (Spreadsheet and Matrix). This object serves as the parent container for multiple Spreadsheet- and/or Matrix-objects and puts them together in a view with multiple tabs:

With folders it is already possible to bring some structure in the Project Explorer and to group together several related objects (spreadsheets with data stemming from text files of similar origin, red, green and blue values of an image imported into three different matrices, etc.). With Workbook the user has the possibility for another additional grouping.