Reorder pages. Redact. Need to make a minor change, and don't have the original. Digitally sign a document. Comment on a document. Join several documents together.
I've also copy pasted scan of my signature on contracts using Gimp in order to not have to print them, it is fairly easy.
For the rest I would tend to ask first for thread original working document if I don't have it already. Having said that I have edited in a hurry an old cv for my partner using libreoffice draw in the past.
Maybe people gere are more picky whom they are doing business with. I mean if I have to sign a contract I'd rather do it with someone who tend to know what they are doing on a general basis.
As much as I agree, I've lost track of how often someone's given me a PDF form to fill out. While driving down to the print store during business hours for a scanner is possible, it's a lot easier to make a sacrifice to the dark gods of format abuse instead.
Well I don't really count "filling out a form" as editing as you don't really change the structure, just add stuff in the blanks and any reader such as the one on yoir browser can do it.
There are a bunch of reasons.