If I read your question correctly, you asked for three instances when drinking an increased amount of water would prove helpful.
The first would be in the case of weight loss. Drinking water helps to stabilize metabolism. Drinking water also helps the person seeking to lose the weight to feel full (and, thus, less likely to fill up on high-calorie drinks or snacks).
People in warm climates or experiencing warm bouts of weather should also increase their intake of water. As the weather warms, we perspire. Drinking an increased amount of water helps to replace the water lost due to perspiration. For the same reason, people who have recently exercised are asked to drink more water as well.
Finally, people taking certain types of medication may be asked to increase their water intake. For example, lithium, commonly prescribed for bipolar disorder, is a salt and tends to cause water retention in patients. Having patients drink copious amounts of water helps to counteract water retention.