Your question has merits. Looking at the screen reader users, they could care less about style, shapes, images. They value good solid text info, content and direction.
It's healthy to question before you do. And it's also OK for the answer we get to be different. Our needs are so different it pays to respect each other opinions and learn from them. Evolution is in how we do things differently.
Greater picture is what we're looking for. If this greater picture happens to be very detailed and very clear, I believe it makes it easier to understand.
And because we can. :) It's not all purpose, it's also challenge.