A square is built by taking a centre/edge block, a corner/edge block, and joining them together.

Our corner/edge pair is ready, but the centre/edge pair is not joined yet. Build that first, then proceed.

Similar situation, but this time the corner/edge pair is in the way. Let's move it out of the way, and then proceed. If we didn't move it, what would happen?

Here again, the corner/edge pair is in the way. There is not much point in putting the corner/edge pair in place early like this, since we must always remove it to insert the other edge.