- Reveals they enemy's Line of Sight.
Omniscience is incredibly useful in the late game. On a random map scenario, enemies usually have resource gatherers scattered across the map, far away from their home towns, plus a Market to bolster their gold-gathering efforts. Using scouts to manually search out the map for enemy buildings and units takes a lot of time and effort, so Omniscience is a great time-saver. Also, if the enemy has been using misdirection to prevent a player from knowing that they are training a mass of counter units, with adequate time, a countering force can be trained.
Although Omniscience is very powerful, it is extremely expensive, costing 100 gold for each enemy unit on the map. The more enemies, the higher the cost, so it may be wise to research Omniscience as soon as possible, since enemies usually train more units as time goes on.
The most powerful of the gods were omniscient -- they could see and hear everything that happened in the universe. Whether they could act on it, however, is another matter.
Tantalus, a Greek ruler and ancestor of Agamemnon, attempted to test the omniscience of the gods by chopping up his own son, Pelops, and serving him to the gods in a stew. All of the Olympians saw through the ruse immediately, except for Demeter, who ate Pelops' shoulder. Pelops was restored to life and Hephaestus forged him a new shoulder. Tantalus was sent to the Underworld, neck deep in a pond. The fruit on the tree above him would always lift when he tried to take a bite, and the water below him always receded when he bent to drink. From his name comes the word "tantalize."