Game Usage
 Protoss vs Terran
Observers are made fairly early against Terran to deal with Spider Mines and to scout walled Terran bases.
 Protoss vs Zerg
Since Corsairs can scout Zerg bases fairly quickly, Observers are used to reveal burrowed Zerg units, mainly Lurkers. Since the Observer is initially quite slow, they are vulnerable to Scourge. It is recommended to upgrade Observer speed or to have multiple Observers accompany a Protoss army.
 Protoss vs Protoss
Observers are usually created fairly early to prevent any Dark Templar harassment. They are also used to scout the opponent's main base.