Гихон
The tunnel leads from the Gihon Spring to the Pool of Siloam. If indeed built under Hezekiah, it dates to a time when Jerusalem was preparing for an impending siege by the Assyrians, led by Sennacherib. Since the Gihon Spring was already protected by a massive tower and was included in the city's defensive wall system, Jerusalem seems to have been supplied with enough water in case of siege even without this tunnel. According to Aharon Horovitz, director of the Megalim Institute, the tunnel can be interpreted as an additional aqueduct designed for keeping the entire outflow of the spring inside the walled area, which included the downstream Pool of Siloam, with the specific purpose of withholding water from any besieging forces. Both the spring itself, and the pool at the end of the tunnel, would have been used by the inhabitants as water sources. Troops positioned outside the walls wouldn't have reached any of it, because even the overflow water released from the Pool of Siloam would have fully disappeared into a karstic system located right outside the southern tip of the city walls. In contrast to that, the previous water system did release all the water not used by the city population into the Kidron Valley to the east, where besieging troops could have taken advantage of it.