Scanners
Building used to increase the scan range to detect further colonies and incomming fleets in the outer space.
Scanners and fog of war
Each 25 scanners will give to you 1 parsec scan range. Scanners are one of the ways to remove the fog of war.
You will need them to detect any movements of your enemies near your territory, remember that any enemy colony can be used as a base to launch an attack against your colonies, if they are very near, you will not have very much time to react. You can't control a region if you don't see what is happening there.
But also, fog of war hide enemy important planets. In a big war, you need to expand your borders and you will need to build scanners in your borders to detect such important planets and attack them to weak your enemy.
Scanners are shared among players of the same empire and among allied empires for this task.
Scanners for detecting incomming fleets
Colonies will detect incomming fleets. The time before the fleet arrive to the planet for detection depends on the size of the fleet (or if there are several incomming fleets, the sum of the size of all fleets, no matter if they come from different planets and arrive at different hours) and the effective scan range of that planet. The more scanners, the sooner you will detect the incomming fleets. Scanners will multiply the standar detection time (colony with no effective scan range) by 1 + scanners / 100.
Scanners effect is shared among near planets that belong to the player or even allies. The effect will get a degradation with distance of the scanner source. You can check the effective scan range of every colony in the "build scanners" screen.