Why is it better than printing to PDF? I just use Print and select Print to PDF as the printer (Firefox on Linux). That actually gives better results because it uses a print style sheet if it is available, whereas this site uses the screen style sheet.
It would be useful if you want to print it exactly as it appears on screen: say as an alternative to taking a screenshot of the site and printing that.
Rare, but I've saved a web page as PDF: the hours of my local library. I keep a printout at my fridge, and the PDF file itself is handy when I'm not online but too lazy/engaged to head to the fridge. I could have saved the page as Web Page Complete, but there's no sense saving the Javascript and Flash junk.