Today both of my cars have bumpers made of plastic and foam blocks. trying to strap or tie off to them involves laying on the ground and feeling for something solid way back underneath the car. Then you have to figure out how to run the strap or rope up through all that plastic and foam without crushing and cracking any of it.
Both of my cars have trailer hitches which gives me a good tie off, but what do I do for the front end? And while I'm at it it would be nice not to have to lie down to find a tie off point.
in my I'ble http://www.instructables.com/id/easy-way-to-cartop-a-canoe-or-boat/ I mentioned I needed a way to tie the canoe down better than a strap through the car doors. this explains how to set the tie downs up without scratching your paint or, cracking any of the plastic on the ends of the car.
I added these tie downs to my minivan last year when I needed a way to tie my canoe on the roof without my great canoe carrier on the back end as I was towing a trailer at the same time. Right after I took these pictures we drove 1 hr on the highway with no problems, the canoe was just as secure when we got there as when we left.
The best part about this is that it is out of sight when not in use, unlike some people I have seen who actually mounted eye-bolts through their bumpers and other parts of the car. With a set of these installed at each door, the hood and trunk you will never run out of good spots to tie off to.