The Observation Post is a Tech Building first seen in War of the Three Powers. Once captured, it reveals a large part of the fog of war around itself as long as it is occupied. Capturing an Observation Post near an enemy base would make a Commander always aware of what the enemy is doing. So control over these structures is often useful.
Observation Posts are only placed on land. However sometimes they're placed on very small islands, so they act almost like if they were placed on the sea.