Excel Services gives users a way to deal with challenges related to sharing and versioning of spreadsheets. Using Excel services, a spreadsheet author is able to save their spreadsheet to a SharePoint document library and give other users browser-based access to the server-calculated version of that spreadsheet.  When a user accesses the spreadsheet, Excel Services will load the spreadsheet, refresh external data if needed, calculate it if necessary, and send the resulting output view back through the browser. Those users will always be viewing the latest version of the spreadsheet, and they will be able to interact with it in the browser. Moreover, the spreadsheet author can limit access to specific sheets or ranges in the spreadsheet, enabling the author to hide intellectual property from the folks looking at the results.  In addition, the user accessing the file in the browser only sees cell values and not the underlying formulas.  If multiple users are accessing the same spreadsheet, it will be loaded and calculated only once by the server, saving the network bandwidth and the wait time for long-running calculations.  In addition, spreadsheet access can be logged and audited. Overall, this provides quite a powerful story for sharing spreadsheets (via the browser) in a controlled and secure way.
Main business value derived from Excel Services is

  1. Sharing spreadsheets through the browser
  2. Building business intelligence (BI) dashboards
  3. Reusing the logic encapsulated in Excel spreadsheets in custom applications