For the data visualizer functionality - the link between Visio and Excel - for Visio to be the starting point, the initial source of the data; to be able to conceive the problem and chart the process in Visio and then instruct it to populate an Excel spreadsheet with the information.
Also for qualifying data for Visio shapes, for example what shows up in the external data window in Visio, allowing us to create an object in Visio in which we can specify that qualifying information for that shape, associate it with that shape and have that qualifying data appear in Excel in the row of the associated shape.