When working on the 250 box challenge, it's very important that you always focus on how the sets of lines that are parallel in 3D space converge towards a single point in 2D space (on the page). You should not be side-stepping the problem of thinking about vanishing points by simply drawing lines that are parallel on the page, and therefore never converging, as doing so will ignore the main focus of this exercise.

We have you draw so many boxes in order to gradually make you more comfortable with the idea that what the marks you're drawing on the flat page represent something that exists in three dimensions. This means trying to capture those convergences, messing it up, then testing them out with the line extensions to see where you were off, then trying again. If you were to just try to keep all your lines parallel on the page, you wouldn't learn anything about how to estimate their convergences consistently.