Simple answer for me is: I have no clue.
Perhaps Honda had originally designed it so that the brake line would be routed through it... and then never changed the a-arm design when they routed the brake line along the upper a-arm instead.
The curvy bit on the a arm is kinda a safety thing. In normal surcumstances the front rim will never touch this parts unless :
A:You try a 450r stem
B:bend a tie rod (badly)or snap a tie rod ball joint.
Also it stops the front tyre hitting the rear of the front a-arm and peeling it of the rim.If you try both A & B.