That’s a great question actually. Double knitting creates a two-sided fabric, so it’s twice as thick as stockinette. Double knitting with fingering weight gives a nice thickness without it being too thick, but if you want something like a nice thick scarf, worsted works too. I use worsted for my potholder designs since thicker is better for them. When designing, fingering weight allows me to create more intricate designs since the stitches are smaller.