I accept that we come to this understanding through our early introduction to language, writing and art. Most people intuitively link structure and presentation when they compose content.

Separating them is incredibly useful. You can see this at a really basic level, in plain text. Lay out a single long paragraph of plain text into lines of max 80 chars, and then insert a word. The layout goes completely wrong and if you don't have tools to help you, you spend ridiculous amounts of time realigning the text. The line-length algorithm you're following (or using) is __totally__ separate from the content. HTML and CSS are this, scaled up to allow fine grained annotation and control of both semantics and presentation, independently.