I was asked this same question by three people this week and it's possible that other people who follow me on twitter (@glycotrainer) thought the same thing but didn't work up the courage o simply forgot to ask for an explanation.
The stomach is similar to a chemistry container. Whatever we put into our mouth enter the stomach and it is forced to dismantle everything in the quickest way possible so it can pass through to the large intestine (for more information about the digestive tract, visit our Gastrointestinal Tract page).
The stomach has two main components to destroy what get into it, acids and enzymes. As soon as something reaches the stomach, it is attacked and degraded by enzymes and acid while the stomach begins to squeeze GENTLY with the hope that it will accelerate the process (just like stirring a glass of salt and water so it dissolves more quickly).
In chemistry, water is used to dilute solutions of all types (including enzyme and acid solutions of all sorts of concentrations). The more water that is added in the lab, the least concentrated and potent each solution becomes.
It's the same thing in the stomach. Water enters and dilutes the enzymes and acids, making the stomach work harder for a longer period of time in digesting what it has to digest.
A lot of people's digestion can be improved by the addition of a fiber supplement, one with enzymes, and a probiotic supplement.