I'd say we do hanmi handachi for the same reason we do suwari waza ... it teaches us to maintain good posture, not over-reach, move with our hips, and move both 'feet', not just one and leave our stance too wide. If we make these errors standing we might not notice (especially if we compensate with muscle) but if we make them seated, it is harder to muscle and we often land on our nose
As for the attacks... I could be wrong here, but I'd say katate tori and ushiro attacks are very common, both against women, or against say a kidnapping target (male or female). Not every attack is a punch to the nose in a bar.