Sometimes the value you pass to a setter can go through additional logic to determine the final value to be set e.g.:

    private int value1;

    private int value2;

    public void setValue2(int value2) {
        this.value2 = value2;                  
    }
    
    public void setValue1(int value1) {
        if (this.value2 > 0) {
            this.value1 = value1 + this.value2;
        } else {
            this.value1 = value1;
        }
    }

Obviously this is a contrived example but if you have logic other than a simple "this.value = value", then you might want to unit test that bit.