Under what conditions can a workbook part ever be null?

I found the following code in a codebase I'm working on:

WorkbookPart workbookPart = spreadsheetDocument.WorkbookPart;
if (workbookPart == null)
{
    // perform some action
} 

I'm a bit confused as to when and under what conditions a WorkbookPart could ever be null. I looked at the relevant portion of the docs but fond nothing regarding this.

When can a WorkbookPart ever be null?