There are a bunch of good reasons to make a habit of getting to your side in half guard. Working from the worst case positioning backward, getting to your side first gives you more options for movement. When you start jiu-jitsu, you are going to get flattened out and you are going to get smashed. Getting to your side first gives you options. Second, your structure can generally bear weight much more easily for a longer period of time on your side than you can flat on your back. Try the “weight bearing drill” to see what I mean. Someone sitting on you flat on your back feels much different than someone sitting on you on your side. You can bear the weight much more comfortably.