There are lots of use cases where you need a PDF document, without letting the user print to PDF it manually. For example sending PDF invoices per email, or generating reports and uploading it to a Dropbox directory.

We use it for a CV portal (think internal linkedin without the social stuff). Advantage for us is in UX (we can have a download PDF button), although I'm not sure it was worth the bother.

That you can script it.

User experience ("download PDF" link). You may also want to send it in an email. (We do both.)