SharePoint 2013 (and previous versions) shipped with Excel Services, allowing Excel workbooks to be viewed in the browser. SharePoint 2013 integrated with Office Web Apps (OWA), which also had the capability to show Excel workbooks in the browser. The complication in 2013 was that the two services had somewhat different functionality, resulting in some organizations having to use Excel Services to gain access to specific BI functionality. In SharePoint 2016, Excel on Office Online Server (OOS), formerly known as Office Web Apps (OWA), has been enhanced and now has the capability to work with Excel workbooks that utilize BI capabilities, such as data models.
Microsoft is now recommending that organizations move away from Excel Services and to use OOS instead. This has a couple impacts on clients:
- Smaller implementations, such as organizations using SharePoint as a platform for Project Server and that support a limited number of users, will now need a second server. Excel services was a service that ran on SharePoint, but OOS is a separate server product that cannot be installed on the same server as SharePoint.
- The list of requirements to support BI is still quite complicated, and will be challenging to communicate to clients. (perhaps a good infographic would help). For example, Excel files with Power View will require SSRS in SharePoint integrated mode to be viewable in the browser. Also, refreshing connections in Excel files with data models will require the PowerPivot add-in.
Microsoft Blog Post about this is here
(see the word doc that is linked in the first paragraph for details on deploying PowerPivot and Power View in 2016)
A good document that compares Excel Services and the Excel Web App from 2013 is here