Why is editing PDFs so prevalent?

Maybe my understanding of the PDFs is wrong, but I've never understood the standard as something that is meant to be edited (beyond signing and form filling). You have your source document that is editable and from there you save a PDF. If I am sending you a PDF I kind of intent for it to be immutable. Yet Acrobat Pro licenses are constantly requested by staff throughout our company because everyone NEEDS to edit PDFs on a daily basis.

What am I, the lowly sysadmin, not understanding about the business side and how they use these documents?