I had originally tried the cable holder from which this was derived and found the opening was just a bit too large for my macbook power cable. Pretty much every time I unplugged, the wire fell out and I had to go pick it up again.

I designed this derivative so that not only is the opening narrower, but it requires intention to get the wire in and out. I can wiggle it around all I want and it will not come out without pulling it along the path.