From Liquipedia Starcraft 2 Wiki
In competitive play Scanner Sweep is commonly used to scout tech buildings, deal with cloaked units such as Dark Templars, Observers or Banshees, scouting for possible expansions, and to scout the opponents army. However, players should always be mindful each scan effectively costs them minerals by denying them the ability to Calldown: MULEs. So it's important to use Scans at the right times, and use them effectively. The scan can be used through the minimap by clicking on it.
 Vs. Protoss
If the player has any reason to suspect the opponent is teching to Dark Templars it's best to save enough energy for one or two scans while laying down an Engineering Bay. The player can also use Scan to kill off the opponent's Observer. Without an Observer, the opponent will have a more difficult time casting Feedback on Ghosts. The temporary advantage gained after killing an Observer can mark a good time to either push out or engage the opponent.
 Vs. Terran
If the player suspects the enemy is teching to Banshees they should save up energy for Scans while making sure they have an engineering bay and preparing to build missile turrets. Besides helping to ward off Banshees, Scan is useful while defending against enemy Ghosts launching Nukes. Scan can also help keep tabs on the opponents largely immobile army, and keep the player aware of when the opponent is moving out. There are also situations where Siege Tanks will be made able to fire on enemy units within range but barely covered by fog of war if the fog is revealed with a Scanner Sweep.
 Vs. Zerg
Around the midgame when a Zerg opponent has recently completed Lair Tech a Terran player often attempts to scout a Zerg opponents tech choice through using Scanner Sweep. If successful this will give the player significant intel which can potentially save hundreds of minerals otherwise used on Missile Turrets in anticipation of a Mutalisk harass. It can also signal the player to adequately prepare for incoming Mutalisk harassment. In anticipation of this the Zerg opponent will often attempt to conceal his tech buildings by building them in remote locations or fake out the Terran player by building a tech building they don't plan on using in an easily scoutable location.
- The Scanner Sweep animation changed from a ring of bright blue lines changing their length rapidly (Beta Patch 8) to a dull orange circle with a radar line circling rapidly counter-clockwise.
- Trump claimed that you should nearly always scan your opponent's base at 6:30 to see what build order they are using. This is especially important for players using a build (such as the 1/1/1) that requires intensive scouting. However, if one can scout with a rax or hellion, this will also suffice due to the opportunity cost.
- Some players will use an SCV early on to build a hidden Barracks near the enemy, after which they lift off and scout the opponent's base. The Barracks will likely be killed while slowly flying over the enemy base, but a scan will be saved for later use, or alternatively the 50 energy saved on a scan can be used for to call down a MULE, which will gather approximately 300 minerals in its lifetime (double the cost of the Barracks, but actually only slightly cheaper when you account for SCV lost mining time).