The ability to insert documents allows for more formatting capabilities without having to directly implement them in Excel Online.
User Story 1:
Bob creates a fancy workbook that requires some level of instructional documentation.
Bob creates a new worksheet and starts merging cells in this complex labyrinth so that he may have a somewhat decently formatted documentation page.
This wastes Bob's time.
He could have done the same thing with a much lower level of friction and time spent by merely creating a Word document with the formatting, then inserting the Word document into the Instructions worksheet.
User Story 2:
Gina receives quotes from her vendors in PDF.
Gina opens the requisition template, which is based on an Excel workbook. There are existing workflows and business processes which pull from cells within different areas of the workbook.
There is a worksheet for quote information.
Rather than retype the entire quote, it would be much quicker and straightforward to insert the PDF on the quote worksheet.
These are already implemented in Excel on both Windows and MacOS; they have been for a very long time. It is, at this point, an expected behavior for a product bearing the name Excel.
Thank you for your work and your time!