1 If you are going to be out shopping, a plain 1 pint tap water enema will clear the rectal vault and sigmoid. No worries about unscheduled visits to the toilet or dealing with flatulence for at least six to eight hours.
2 If you are driving more than an hour or two to visit or be a tourist, 3 -4 pints of tap water will usually clear the transverse, descending and sigmoid colon along with the rectum, allowing at least 24 to 36 hours without the need to deal with flatulence or need to evacuate in poorly maintained public restrooms.
3 You ate food or drank something which is causing gastric distress and need to ease abdominal pressure.
4 You haven't cleared the lower gut in a couple of days and are experiencing discomfort. You do not want to deal with the 24 to 36 hour drizzles that go with oral laxatives such a bisacodyl, milk of magnesia, citrate of magnesia, senna etc. Again 2 pints of clear tap water provides fast relief without being left to wonder if what needs to pass later is actually gas or legacy effects of the laxative.
5 You are scheduled for an annual physical exam. Your physician will appreciate not encountering mass and gas during the DRE.
6 You are scheduled for surgery and will be hospitalized for more than one day.
7 You don't want to asphyxiate your companion during intimacy or try to find comic relief when flatulence produces obnoxious noises.