Stimulant
We all know that coffee contains a certain amount of caffeine. Some products obviously contain a little more than others; it depends on the brand and the method of processing the beans. But no matter what kind of coffee (even decaf) you brew, caffeine is going to make up some part of your cup. This well-known stimulant can prompt the contraction of the muscles in your digestive tract, including the colon and even the sphincter. When the contractions are invigorated, so to speak, your body, or at least your digestive system, will move the waste until it is expelled, producing a bowel movement.
Diuretic
While coffee can help to stimulate a bowel movement, it isn't necessarily a treatment for constipation. Since coffee also works as a diuretic, which essentially means that it can remove some liquid from the body, it can leach liquid from a stool and further dry out it out, making it even more difficult to pass. So if you are constipated, drinking coffee could ultimately worsen the condition. However, the diuretic nature of coffee isn't enough to cause constipation. It can still prompt a bowel movement in someone who is "regular."
Other Benefits
Besides keeping you awake (and possibly prompting a bowel movement), coffee consumption actually has other health benefits to the average daily drinker. Coffee does contain some antioxidant properties, produced through the roasting process of the beans. It may also help reduce the risk of both Alzheimer's and Type 2 diabetes. There is some evidence, according to the Harvard Medical School, that coffee even has some anti-cancer properties.