When losing, or maintaining weight the one thing which everyone seems to agree upon is the notion of slowing down the pace of eating. I suspect that the success of the glass of water with the evening meal is due to two reasons.
Naturally we need to satisfy our thirst all the time, but this particular glass of water has little, or nothing to do with satisfying thirst.
The water takes up some space in the stomach, so as a consequence less food is needed to feel full than would otherwise be. That really matters when the meal is lacking in high caloric yumminesses. At the very least I want to leave the table thinking I did have my evening meal and that my hunger has been satisfied.
The other consequence of the glass of water with the evening meal is a bit more subtle. Have you ever timed yourself swapping the knife for the glass, taking some sips (or gulps) of water, placing the glass back down on the table (preferably in the upright position) and picking up the knife again?
I bet that you have not timed all this. Certainly I haven’t either. It can not be minutes can it? Nanoseconds, seconds? Regardless of how ever long the process takes, it does slow down eating!