Reasons why it's convenient to knit by using a smaller left hand needle (or opposite if you're a left handed knitter!): 1. easier to insert the needle in stitches; 2. stitches have easier time sliding on the needle, especially useful for tight, dense fabric knitting; 3. no need for two sets of needles when knitting both sleeves, socks or mittens at the same time. It's basically a small knitting hack to increase the convenience of knitting.