Maybe. . .
Back to the original question, about why someone might tie a line from the bow to the rack. I haul my boats on a half-ton rated utility trailer, with a custom rack system made of 1.5" square tubing (translation: home-made). Not much chance this rack will fail, and the boats are back in the draft of a GMC Suburban, at least until we get over 4 boats. Given all the discussion about oil-canning, dents, and the negative effects of over-tightening hold down straps, I have eased up on the strap tension. This does increase the concern about boats shifting fore-aft under the straps. Lines from the bow and stern, back to the rack, limit this potential fore-aft shift. If the subject in question had his/her boat anchored with the front strap close to the center of the boat, he/she may have had a similar concern about the boat “squirting” forward out of the straps.
OBTW, I am an advocate of bow/stern lines when hauling boats on top of a vehicle. Personal preference, on a truck or Suburban, is the straps from under the hood along with the rear hitch. Suggest that people strive for a trapezoid, rather than a parallelogram, cross section. I always used white nylon line with tautline or trucker hitch, and never noticed a vision problem. For long hauls on the trailer, bow and stern lines still work for longer canoes, but it seems more effective to double-strap kayaks and shorter canoes.