The long sashiko needle is one of the most typical tools in the technique. For those used to regular embroidery, it may seem unusual at first.
The reason for the length is the way you sew. In sashiko you often work along longer lines in the pattern and pick up several small stitches on the needle before pulling the thread through the fabric.
By grouping multiple stitches on the needle, you save time and can work at a more fluid pace. It also often results in more even stitches because your hand moves in a calm, regular pattern.
The long needle not only makes the work faster but is also an important part of the technique itself.