The Marine is the basic unit in the Terran force. Equally effective against both air and ground enemies, this means a group of marines is a good force against almost anything. Groups of marines can use their rifles to take down small squadrons of light airships quickly. Consider the marine an alternative to the Missile Turret whenever building one is prohibitive. Using Marines as mobile Missile Turrets, you can deal a heavy blow to ground and air units alike, so long as they travel in large groups.

Although Marines can take a decent fight to enemy units, they really shine on the ground, stacked by fours in a Bunker. This is especially true in levels where bases are hit with wave after wave of enemies. A Bunker full of Marines with upgraded range and attack values can mow down small groups of enemies indefinitely. A grid of Bunkers is virtually impenetrable.

Marines are typically built anyway as a defensive measure early in the game. As the game goes on and the Terran player gains access to more powerful units, its common to send the Marines with a few Medics to the enemy. If they win the fight, then thats one base taken care of early in the game. Even if they lose, the player usually has some intel about the enemy's location and defense force they are using. Plus some supplies are freed allowing the player to build more powerful units.

Even in long games Marines are still used as an emergency defense. If the player happens to know of an approaching attack force, then Marines can be quickly built and hold against most enemies. However infantry tends to be vulnerable to high armored units as well as enemies with splash attacks.