The purpose of this exercise
This exercise removes the direct consideration of vanishing points, and brings our attention back to the things we're actually drawing, which exist within the page or canvas we're working on. In other words, it helps us focus more on the lines and how they converge on the flat page, instead of always having to go back to a vanishing point for everything. We learn to pay attention to neighbouring edges, and to identify such key points that we can use to our advantage.
This is incredibly important when doing our own work, for the simple reason that if you're constantly trying to find whichever vanishing point you need to be paying attention to at any given time, your brain is going to be dropping in and out of (or more accurately, never actually slipping into) the flow state where you really make your best creative decisions. Your focus should absolutely not be on how the lines that need to be drawn get drawn - you need to be focusing on what it is you want to convey on the page, what it is you're actually trying to draw.
I've mentioned this before, and I'll probably mention it a lot more throughout the course - Drawabox is all about forcing students to think hyper-consciously about how they're approaching every mark they make, so that outside of the course, you don't have to think about it at all. We fuss over every last little thing here, so that when you're drawing your own work, you can rely on your instincts with confidence, and focus only on what it is you wish to draw.