[defensive7132]

How come we say drawn line has 0 width and using paper instead of drawn line has non 0 width? I can say a paper has 0 width as well. But if we are to stick to physical world, then neither drawn line nor paper has 0 width.

You can't just treat things differently just because.

If your point is to obstruct view of an object, knowing objects dimensions we can calculate it's position behind the paper wall/line.

[ganzuul]

The point is to see what we can still calculate when we change our assumptions.

In the case where we construct a black-on-white boundary with two sheets of paper we could for example say that the white paper on its own has two-dimensions but the combination of two papers has fractal dimensions, like 1.5 dimensions. This eventually leads to the Mandelbrot fractal.

But note that we took another step after that, similar to the first. This third construction let us very quickly start to think about highly abstract things and our minds remain capable of dealing with them. We can 'see' otherwise invisible things with our minds, like reading between the lines.