Defend your base against incoming enemies that attack with rockets, parabombs, jets and helicopters. Make sure no one breaks through your line of defense!
A better example of a piercing shot. A minor change in this case is also that the piercing spawns an explosion as if the missile exploded. This is important because normal explosions include bonuses from impact speed and the explosion radius upgrades that the player activated.
If the piercing effect wouldn't include this, it's pretty annoying if a wave of attackers isn't wiped because the rocket pierced the enemy instead of just exploding.