I can imagine one use for the horizontal line group: when there are athletes coming towards the camera, in case I want to focus on the eyes of the closest subject, I could use this. If I use regular group it tends to catch details in the chest (shirt with athlete's number, advertisements etc.) because they are such good focus targets and because I can't move the regular group area all that much off center along the short axis of the frame (for vertical compositions regular group can work great but in horizontal / landscape compositions it can be moved only one step up or down from center). With the horizontal line I could place the line at eye level (top row) and just let the camera choose the closest subject.
Another option could be when photographing race cars, to place the focus on the front of the leading car without risking focusing on the road. At least I can imagine it being useful for that.
It will be interesting to try these out.
