Why do Terran players bring SCVs with their main army so often?

Solution 1:

In StarCraft 2, units will automatically target the closest attacking enemy unit. This is a change from Brood War, where worker units were ignored by the targeting AI.

This allows players to use workers to tank damage, whereas otherwise they would just be used to clog space. Since Marines have very high DPS, but low health, they are very strong when kept alive. Since there is a barrier of SCV's soaking up enemy hits during these attacks, they live longer and have significantly more damage potential. It is difficult for opponents to focus down Marines without exposing their units to being surrounded by the SCV's and then killed off.

Other races have other units that fulfil the often necessary role of soaking up damage- Protoss can use Zealots for the same function (for much greater cost-effectiveness than Probes) and Zergs can use both Zerglings and Roaches, depending on the stage of the game. Terran doesn't really have a unit like this except the Marauder, which has a high cost due to the necessity of a Tech Lab, gas, and production time. A Terran is always going to have plenty of SCV's, so if he sees an opportunity to gain an advantage by sacrificing them, it's worthwhile.

SCV's are also naturally tough- they have 45hp (which had to be reduced from 60 early in the game because of exactly this phenomenon) whereas the other races' workers have only 40 converted health.

Much later in the game with Mech armies, Terran players will bring SCV's for repairing, but these are generally much slower and more deliberate assaults.

Solution 2:

Here is a listing of different aspects why SCVs can help an aggressive player

• In direct combat (without micro) they are the strongest workers with their 45 HP.
• They can repair each other.
• They can build bunkers (and other buildings) for marines and repair them.
• They block melee units from attacking the squishy marines.
• They can surround units like Stalkers preventing them from running away.
• Terran can compensate worker kills best. They have Mules which work best with low worker counts for both players. So if you trade workers (1 SCV for 1 Probe) the Terran is better off.
• In a base race scenario the safest place is at the front and Terran can lift important buildings to win the base race.
• SCVs do not prevent the Marines from attacking, but Probes would block Zelots and Drones would block Zerglings.

Solution 3:

Because terran has no melee units. Z and p require their first units to close the gap (lings and zealots respectively), and later these become the "meatshields" to their army, so pulling probes/drones is unneeded. Terran has no melee units, so they use scv's to tank and keep their units safe at range and away from p/z melee units so they don't inflict damage and you do more overall.

Solution 4:

Like fbueckert commented, Terran SCVs can repair mech units. This allows those units to live longer and continue fighting. Probes and Drones would be pointless because they do not have the same benefit of being able to repair.

An alternative to bringing SCVs is to call down MULEs from an Orbital Command Center. These have the benefit of also being able to repair, and are available anywhere on the map (provided you have enough energy to call for one).