An archaeologist and a hydraulic engineer from Penn State have made a fascinating discovery at the Mayan city of Palenque. Apparently at least one of the aqueducts built by the ancient Mayans to carry water into the city was designed to increase water pressure, narrowing along its length to maximize the pressure at the bottom end. You can read the University press release here (or in virtually unaltered form in any of the news stories about the discovery). This seems to be the first known Mayan waterway making use of hydraulic principles to control water pressure, and suggests they had a much better understanding of the subject than anyone realized.
What was it for? Well, there may have been a practial reason -- perhaps the water went up again into some now-vanished structure in the city. Or maybe it was a fountain. Underground wells and waterways were vitally important to the Mayans, and had religious significance as well. A jet of water shooting up 20 feet from the ground would be really impressive, and the height of the fountain would be a good way to tell how much water is available for irrigation in a given season.
Or maybe the Mayans built the world's first water theme park. They were a very advanced civilization, you know. Had to pass the time somehow while waiting for the calendar to run out.