In the single-player campaign, the SAM Site is the Soviets' only viable defense against air units, as the Rocket Soldier is available only in skirmish and multiplayer, and the Mammoth Tank's missiles are not effective against helicopters. Even so the SAM is nowhere as lethal as the Allies' AA gun, and you'll often need many of these in order to fight off Allied Longbow raids. Conversely as an Allied player, you can take advantage of this lack of lethality and use groups of Longbows to destroy SAMs and move on to whatever they're protecting.
Compared to the Allied AA gun, the SAM Site deals half the damage output and has slightly greater range. It is a bit more expensive, but also consumes much less power, and is not dependent on power to function.
It cannot harm ground-based units, or interfere with them in any way other than potentially distracting them from attacking a more valuable target (e.g. the Construction Yard).
It was replaced with the flak cannon in the timeline leading to the events of Red Alert 2 which was similar to the AA gun. In the timeline leading to the Tiberium universe, Nod continued to use a modernized version of them.
- Anti-aircraft gun, Allied counterpart