It's to make the boarding process smoother. Since the person in the middle seat is Group 4, then your companion in the aisle doesn't have to get up and get out of their way in order for them to get into your seat. Keeps the aisle unblocked as well.

Is this a domestic or international flight? If domestic, overhead bin space is a non-issue. For international, it's also usually been fine for me on ANA. It's not nearly as competitive for that space compared to flights on other carriers.