Controls the amount of ease build into the pattern. Since these are leggings that are to be made of stretch fabric, the ease is negative. So the completed garment will be narrower than the measurements would dictate.
Getting the ease right for your type of fabric is essential for getting a good fit. If you make the ease too negative, the leggings will be too constricting, and limiting movement. If you don’t make the ease negative enough, the leggings will not hug your legs and torso. The fabric should be stretched in all places, without being too tight.