This node writes the input data table into a spreadsheet and appends it to an existing workbook. If no workbook is present a new one will be created. The files can be read then with other applications like Microsoft Excel, for example. The writer supports both the old 'xls' format as well as 'xlsx', whereby the same limitations apply as described in the documentation to the XLS Writer node (max 65536x256 for 'xls').
Note that if the destination location is a remote URL not all options are available because in general it's not possible to determine whether the remote location exists. In this case it will always be overwritten.