The double increase can be used to create a decorative column of raised stitches.
Work in pattern up to marker, pick up stitch from below right hand needle and work in pattern. Move yarn to back of work. Move marker. Slip next stitch. Pick up loop from below stitch on left hand needle and place on left hand needle. Work in pattern into back of picked up stitch.
Knit into the front, then back, then front of the next stitch on the needles, making 3 stitches from one.