From Liquipedia Starcraft 2 Wiki
The Missile Turret (or Turret for short) is the Terran's anti-air static defense as well as one of the Terran's detectors. Missile Turrets require an Engineering Bay in order to be constructed, which also has upgrades for the Missile Turret: Hi-Sec Auto Tracking and Building Armor.
The Missile Turret's attack is about as powerful as four Marines. The twin attack (12x2) makes it less effective against high armor targets such as Battlecruisers but despite this setback, Missile Turrets still deal more DPS per cost than the Photon Cannon or Spore Crawler. Although they cannot out-range Carriers, the Missile Turret's high damage and endurance is still very potent for clearing out Interceptors.
 Competitive Use
 Vs. Terran
A few well placed Missile Turrets can fend off Banshee harass in the early game. Since a Missile Turret can detect farther than it can shoot, it is still useful when the Cloaked Banshee is out of range. A small group of stimmed Marines can run up to a detected Banshee and shoot it down if the opposing player does not pull back immediately.
 Vs. Zerg
A handful of Missile Turrets can effectively hold off large numbers of Mutalisks and is recommended for protecting your vulnerable SCVs. When under attack, your SCVs can repair the Missile Turrets, increasing its longevity. However, this guarantees that the Mutalisk's Glaive Worm attack will bounce from the Missile Turret onto the SCVs.
 Vs. Protoss
Missile Turrets can minimize damage taken from Dark Templar, and can also be used to ward off Phoenix harass. They are decent against uncharged, lone Void Ray attacks, but fall extremely quickly to multiple charged Void Rays. They are a strong choice to defend from Oracle harass, placed in close proximity to mineral lines.
 Patch Changes
|Patch 7 (version 0.9.0.14621B)|