Fat coats gluten strands and blocks them from bonding to each other. Add butter at the start and you're trying to build a network through a layer of grease - hand kneading would take 50+ minutes and your arms would file a complaint.
The course method: knead to about 70-80% gluten development first, THEN add soft room-temperature butter, then 2-3 more minutes to full windowpane. The network is already built, the fat just enriches it.
And yes, the amount changes the bread meaningfully. More fat = richer, softer, closer, more tender crumb (think brioche territory). Less = leaner and chewier (think baguette direction). Butter brings flavour, neutral oil keeps the crumb soft for longer. Pick by what you want the bread to BE.
