The only reason for this, I believe, is to ensure that you can eat "enough" at a meal.
Pre-op I'd drink tons during a meal. This probably helped me cut back on the volume of food - God knows how big I'd have been if I'd not been a big diet soda drinker during meals.
Post op, I have only once taken a drink during a meal which I painfully regretted. I've managed to adjust but I miss cleansing my mouth during a meal, as I partly did it so that the next bite of food tasted better.
My wife is 7 years out from hers and she sips during a meal but when I met her 6 years ago, she couldn't so much as sip. I still remember our first date - she could barely eat a thing, couldn't drink at all and was really embarrassed as the waiter was so concerned that the food wasn't good!