Is there any necessity to pass a variable parameter to a method while the variable declared global?

Question

I am writing a class in java of Monte-Carlo algorithm. Here is the written code -

public class MonteCarlo {
    int[][] matrix;
    public void monteCarlo(List<Node> nodeList) {
        matrix = new int[nodeList.size()][nodeList.size()];
        initMatrix(nodeList, matrix);
    }

    private void initMatrix(List<Node> list, int[][] matrix) {

    }
}

public class TestMain {
        public static void main(String[] args) throws IOException{

        GraphGenerator generator = new GraphGenerator();
        List<Node> nodeList = generator.graphGenerator(50, 11000, 6000, 40);
        MonteCarlo monteCarlo = new MonteCarlo();
        monteCarlo.monteCarlo(nodeList);
   }

}

In here initMatrix method i am passing two parameter. But the matrix array is globally declared. I personally sometime found that is helpful to keep track which method is responsible for what. But is it a bad coding practice to pass a variable parameter even it is declared in globally.