Originally Posted by
ThermionicScott
They're mostly useful when you're trying to pump up a tube that doesn't have enough pressure to hold the valve out. Just squish the tire with your thumb to hold the valve out and in place.
IME they do a lot more harm than good. If I had a dollar for every time I saw a newbie at the coop tighten the nut to the hilt then pump the tire up and wonder what the lump was in their tire was without realizing the tube was so tight the bead couldn't seat properly... I'd have like $25.