Excel files has long history
Excel 97-2003 workbook:
This is a legacy Excel file that follows a binary file format. The file extension of the format is .xls.
Excel 97-2003 in terms of apache poi is called - Horrible Spreadsheet Format As the Excel file format is complex and contains a number of tricky characteristics,
apache-poi jar has code to handle these file

Excel 2007+ workbook:
This is the default XML-based file format for Excel 2007 and later versions. It follows the Office Open XML (OOXML) format, which is a zipped, XML-based file format developed by Microsoft for representing office documents. The file extension of the format is .xlsx. ( DOCX,PPTX are other OOXML based examples).
Excel 2007+ workbook in terms of apache poi is called - XML Spreadsheet Format -these file format are advanced version of HSSF and has additional features, code to handle these files are written in apache-poi-ooxml jar

As .xls is almost dead but still some applications use it, so for backward compatibility both dependencies are required.
here is what Apache have to say -
- HSSF Excel XLS poi For HSSF only, if common SS is needed see below
- Common SS Excel XLS and XLSX poi-ooxml WorkbookFactory and friends
all require poi-ooxml, not just core poi
you can read more at their official website http://poi.apache.org/components/index.html#components