Hi Amber,
Thank you for using Syncfusion products.
Either a stream or a filename can be used to open a workbook, as both does the same functionality which returns a workbook. Internally the file name creates a stream and processes its data. Whereas, if a stream is given as input, it processes from that stream itself.
The stream option is given in addition for user's convenience to use the stream wherever necessary. If a user is not able to manage the code with streams, then can use a file name to open.
Please let us know if you need further clarification.
Regards,
Johnson