Why there are things that, for example, block other things when watching TV (2d TV) and where they are. Why they are all "in the same direction"...
This is the first, simplest, off-hand example I can come up with.
Things are blocked "in parallel" (assuming the camera is far away from the objects) because that projection is a linear map (I repeat, assuming "far away focus") and has a one dimensional kernel: 2d + 1d = 3d, which means (flat TV + orthogonal lines = 3d world), that is the dimension formula.
This is the first, simplest, off-hand example I can come up with.
Things are blocked "in parallel" (assuming the camera is far away from the objects) because that projection is a linear map (I repeat, assuming "far away focus") and has a one dimensional kernel: 2d + 1d = 3d, which means (flat TV + orthogonal lines = 3d world), that is the dimension formula.