Ask yourself. When you are an assistant referee and the ball is on the other side of the field, how much attention do you pay to the ball/play versus the offside line in your half of the field? You know you need to be aligned at all times. But you know the ball is unlikely to come to your half of the field.
But incorrect alignment can be costly. Not paying attention to the offside line can easily leads you to not know who was onside and who was offside at the moment of the last touch. Watch the video below. An unexpected ball forces the AR to make an offside position. Unfortunately, he was not aligned. He was watching the ball and not the offside line. So he ends up making an incorrect decision.