The Armored Personnel Carrier (APC) is a heavily armored vehicle. It can effectively shoot down most infantry without receiving significant damage, excluding rocket soldiers and shock troopers. For larger blobs, consider crushing instead (see below).
The APC is also good for early game scouting thanks to its speed. However, the allied Ranger is a superior early game scout. It is both faster and can see further.
The APC can be produced early in the game by soviets. A player can carry 5 rocket soldiers, grenadiers, or flame throwers in an APC and infiltrate the enemy base, in an attempt to destroy some buildings. Harming the economy in the early game by destroying Ore Refineries or Ore Trucks can be fatal to the opponent on the long term, and since the APC and the above units can be produced early, this tactic can be employed early in the game.
For larger blobs, crushing enemies by manually driving over them is far more effective than shooting them. The APC's speed can make it crush many enemies before getting destroyed. Multiple APCs can be used to maximize the effect.