Observers are the protoss mobile detectors. Although observers are 'permanently' cloaked, protoss players must also be alert to enemy detector units; the observer's low health gives little time to escape attackers. Observers are cheap enough to have a handful accompany an attack force, especially against a target with many detectors. The Gravitic Boosters upgrade is particularly useful in this accompanying role. They are useful against Dark Templar, Spider Mines and Lurkers in particular.
