PDF, as it evolved from PostScript, is the de facto standard for most print jobs. Commercial print (think magazines and flyers), packaging, large format (e.g. billboards), books, many textile prints, etc. They all use PDF extensively. And very often those PDFs are print technically garbage. Fixing that in the original application is either not possible or, more frequently, requires knowledge the designers simply don’t have. So the print shop’s prepress department does it in PDF directly.