What if someone wants to use an Excel feature that isn't available in Excel Services? For example, someone might want to use the AutoSum button, apply their own filter buttons or print using Excel's printing capabilities.
They can do all that by opening a snapshot in Excel. A snapshot is a limited version of the workbook data in Excel.
The snapshot does not contain formulas, data connections, conditional formatting, hidden cells, rows, columns or worksheets, private information such as workbook author name, and filter buttons.
Working with a snapshot in Excel does not in any way alter the data published to your SharePoint site.
To open a snapshot from Excel Services, click Open, and then click Snapshot in Excel.