Justin L.wrote a review of Thule Hood Loop Strap on January 28, 2013
Familiarity: I've put it through the wringer
Even if you have a rack on your vehicle, the rack is not meant to have all of that force on it. These are perfect for tying down the front of the kayak or canoe. I never put my canoe on the roof without using these. I have seen roof racks ripped off the roof of vehicles from the forces of wind puts on your canoe/kayak.
When your done they can be closed under the hood of your car/truck so you can leave them installed for whenever they are needed. If you want them off for some reason, they come off in the time it takes to unscrew a screw.