LibreOffice doesn’t format all Word documents the same. They’ve done an amazing job, but AFAIK, they haven’t solved every quirk. Also, if you use Excel, Calc doesn’t support everything from VBA. So esoteric VBA macros won’t work sometimes.
Don’t get me wrong, I’d love for LibreOffice to become better, but having Office on a second install is also a sortove “insurance” for when LibreOffice doesn’t work 100% on old documents.
(The Office XML based file formats are a lot better here. I’m mostly talking about the old OLE-dump pre-XML formats)