Besides providing a safety blanket if one of your other tie downs should fail the bow and stern line play another important role in keeping your rack attached to your vehicle. Over the years here at WCK we have seen several cases where a boat comes off a vehicle while still firmly tied to the rack. This is because the rack can actually pull off the vehicle, especially some factory racks. It's also worth noting that most after market rack manufacturers (ie Yakima & Thule) clearly state in their warranty information that your rack warranty is void if you are not using a bow and stern line while transporting your canoe or kayak.
Having the bow and stern of your boat tied down to the bumper or tow points on your vehicle moves all the upward pressure on your rack down to the frame. 9 out of 10 times that a boat comes off a roof there is no bow or stern line attached.
To make securing your canoe or kayak to your vehicle easier we are now stocking the Seals ratchet tie downs. These easy to use one handed ratchets are probably the simplest and quickest way to attach a bow and stern line to your vehicle.