When you screw the nut down so the stem is tight before you get the tire in the rim there isn't enough room for the tire to fit properly over the tube. If you leave the tube loose, without tightening the nut down you can fit the tire into the bead properly with the tube "up" and inside the tire. Then, after inflating, tighten the nut. Normally tightening the nut does nothing and if you keep your tires inflated perfectly it probably makes no difference, but if you let your tire get a bit low on air and you have occasion to brake crazy hard you could conceivably cause the tire so rotate and damage the tube by putting force on the valve. Having the valve screwed down diminishes the chance that the valve will come under a strain like that. It simply holds it in place. Lots of people ignore their use.