OK, if you are knitting a swatch and you are going to wash it, or you are going to handle your knitted piece a lot, Just leaving open stitches isn't practical you'll want to secure the stitches.
Running the tail of yarn through each stitch is a fast way to secure them.
Sometimes you have a very stretchy, open piece of knitting. Using a regular bind off method may distort the piece. Use this method to temporarily secure the stitches and avoid any distortion.
This is also a good technique for gathering the top of hats. Just run the tail through the stitches, pull tight and secure ... instant gathers!