It's important a) because it is true to life (many young soldiers really did this) and b) because it shows just how desperate the soldiers were. Here they were, trained members of the American military, and they were so unhappy with their situation and the war they were fighting that they were willing to shoot themselves to escape it. That's pretty bad. Since he also shoots a baby animal, not for food but just to hurt it, we can also assume that one is symbolic of the other: to live through Vietnam you have to hurt innocent things.
It also symbolizes a major change in Rat's character. Though he took out his aggression on the baby water buffalo after his best friend Curt Lemon was killed, Rat was typically a happy go lucky guy. He carried comic books with him. When Lee Strunk emerged from the tunnel, rat began cracking jokes. "Worms, Rat Kile said. Right out of the grave. Fuckin' zombie." (12) and Rat and Curt had played games like boys to pass the time. He choice to shoot himself in the foot shows how the war can weigh on men and what they're willing to resort to to remove themselves from the killing.