In the modern game, it is necessary to move back and across. One can move slightly back and across before placing more weight on the back foot while ensuring that the head is kept forwards.This gives several advantages. First, it gives the batter a good indication of the position of the off-stump. With the modern emphasis of bowling in a channel on and just outside off-stump, a batter must have some means of knowing when to let the ball pass to the wicket keeper. Second, the batter is already in a partial back foot position. Short-pitched fast bowling is the norm today. Therefore, it pays to be biased towards the back foot position before the ball is delivered. From this position, the batter can move further back if the ball is short without compromising the ability to move forward if the ball is pitched up, because there is extra weight placed on the back foot. Third, for most deliveries that are played on the front foot, the front leg has to travel slightly to the left of the line of the ball. By moving back and across, the front foot is inside the line of the back foot. This tends to minimise the effect of the front foot moving too early, which usually results in the front foot moving outside the line of the back foot, and unless the ball is pitched on off-stump or wider, causes the batter to play across the front pad. This increases the likelihood of an lbw dismissal. Fourth, the batter is in a more balanced position because the area of base support is increased. Fifth, leg-side play is enhanced because the front foot is on the left side of the back foot, providing the ideal position from which to play balls on or around leg stump. And, finally, the elevated hip line (right lateral pelvic rotation) is a source of stored energy, which can be released during the downswing to generate bat speed.
Not my words by the way.... Past Sri Lankan Batting coach