Convenience is exactly it.

Have to admit I find it pretty damn useful myself, even though I believe PDFs to be completely out of scope of a web browser.

I really wish the format would disappear. Everybody says it's useful at being a "perfect digital representation of what things look like on paper", which is certainly super useful for printing. But I cannot understand how so much information that should simply be distributed in either HTML (forms), epub (ebooks) or jpg/png (scans and digital images) is distributed over PDF, a pain-in-the-arse, overengineered format whose readers are always extremely slow and riddled with security issues and from which it's extremely hard to extract source data.