When learning to bat, one is told to bat side-on, so that the stance is horizontal to stumps -not sure whether it is the correct method/teaching, perhaps it has to do with showing and not covering the stumps, or evading the bounce!.
However with side-on stance it is not ease to judge the length of the ball.
Judging the length of the ball is key to batting and when you are batting side-on, the distance seems either too far or too close. There are cricketers who bat open chested but they basically have learned to bat side-on. However they balance their technique to adapt to various bowling. Balance is a degree of control a person has over their body.
Side-on and open open-chested : When batting side-on you need to turn your top half of your body to judge the length of the ball. With regular training batsmen do adapt, however if the surface is having low bounce than they find it difficult to make assumption of the length.
When you are open chested your footwork is more balanced as the weight of your leg is evenly distributed without much effort. Here a batsman has more time to judge the length and deviation of the ball. It is much easier to play the horizontal shot(cut and sweep shot in particular) and also the vertical shots(flick and on-drive in particular)if you are open-chested.