When opening a workbook in SharePoint using Excel 2016, it opens as read-only with the following message:
We opened this workbook read-only from the server.
To edit the workbook, click on the button Edit Workbook
This is a design change from Excel 2013, which opened workbooks directly into edit mode by default. The reason for the change in Excel 2016 is to prevent locking out users from editing the file.
When you open a file in edit mode, you get exclusive editing capability in the file, and no other person can then open the file to edit. They can only view the spreadsheet in read-only mode. Even if you are the only person that uses the workbook, this can happen when you open a file in edit mode and leave it open on your work machine, for example, which means you are locked out of the file for editing on another computer at home or elsewhere.
Changing the behavior to open workbooks by default in read-only mode helps prevent these scenarios. Many users read information but never make edits to the spreadsheet. For these users, opening the file as read-only works just fine for them, and it leaves the file unlocked for editing for someone else. So this change in behavior helps keep the workbook available for those people who need to edit.