Is @FunctionalInterface necessary in jdk8

Question

See this program, I used Lambda and @FunctionalInterface with Java 8. It works well even if I remove the @FunctionalInterface annotation, so I was confused is it necessary to mark @FunctionalInterface on the interface? Why some internal interfaces marked as @FunctionalInterface??

@FunctionalInterface
public interface CalcInterface<N, V> {
    V operation(N n1, N n2);
}

public static class NumberOperation<N extends Number, V extends Number> {

    private N n1;
    private N n2;

    public NumberOperation(N n1, N n2) {
        this.n1 = n1;
        this.n2 = n2;
    }

    public V calc1(CalcInterface<N, V> ci) {
        V v = ci.operation(n1, n2);
        return v;
    }

}

public static void main(String[] args) {
    NumberOperation<Integer, Integer> np = new NumberOperation(13, 10);
    System.out.println(np.calc1((n1, n2) -> n1 + n2));
    System.out.println(np.calc1((n1, n2) -> n1 * n2));
}