As its name clearly says, this tool was created with the sole and plain purpose of helping you convert the newer default format of Excel files, XLSX, to the one that was supported by the older versions, up until Excel 2007, the XLS format. At this moment, you cannot load and use XLSX files using the older versions of Excel, but you can use this tool and convert them to the compatible XLS format.
The “Excel XLSX to XLS Converter Software” is very simple and straightforward. Its interface is very plain, allowing quick access to all the features and options of the app. There are anyway very few, basic functions. The most important feature of this tool, I'd say, is the fact that it lets you batch convert multiple files at the same time. You can also choose where to save the output files (a local folder). There aren't any other remarkable functions. The conversion speed is neither great nor disastrous, it's rather average.
Besides the lack of features, this tool also fails to work properly on systems where Excel is not installed. Another thing that bothers me about it is that it seems quite pricey to me, especially considering its utter simplicity.
Anyway, if you really need to convert XLSX to XLS and you lack any other options, then (and only then) you should also consider trying this tool.