[POiNTx]

Does this allow external programs/servers to interact with a workbook (xlsx file)? Or is this just to run more complex calculations inside cell(s)?

There's UNO and there's also openpyxl to interact with workbooks from Python, but both solutions were a bit slow when testing it.

There's a lot of value in having existing workbooks which a lot of businesses already use, and extending them so they can be interacted with or queried from a server. For example to allow further access from a webapp without allowing changes to the entire workbook or hiding the full dataset.