Like shooting a rifle with a scope, the squint helps concentrate all the focus of your best eye on that elusive hook eye target. As in making a shot with the rifle, the squinter needs to line up the tippet and fly ring eye, take in a deep breath, let a little out and hold it steady. Then you make the stab of tippet to hole. Now the Orvis Company makes a little device that you can use to thread fly leaders into hooks without the need for squinting or illegal utterances. These are, of course, for wimps and wussies.
Real men squint.
If you see someone on a trout stream scrunching one eye closed, he or she is no doubt midge fishing.