Vents are typically used in feeding bottles to allow air to enter the bottle as the baby drinks to prevent a vacuum from forming inside the bottle, making it increasingly difficult to get milk out of the bottle.
Since Twist Pouches are flexible (unlike regular bottles), they collapse as your baby drinks. The bag shrinks down as the volume of milk decreases, and no vacuum is formed as the milk is depleted. The amount of suction required is the always same regardless of milk level, and no air needs to enter the pouch to help push the milk out. This eliminates air from the pouch.