Variance is really important to pass generics to other functions.

You are wondering how would `List` in java look like if it would not support variance? Just check out arrays :)

Imagine you have the array of integers and you would like to change the first item:

void test() {
  Integer[] array = new Integer[10];
  replace(array);
}
void replace(Number[] numbers) {
  numbers[0] = 1.0f;
}

This code would result with Integer array that contains both ints and a float (not good, right?).

Because `List` takes variance into account, trying doing the same thing:

void test() {
  List<Integer> integerList = new ArrayList<>();
  replace(integerList); // Compiler error
}
void replace(List<Number> numbers) {
  numbers.set(0, 1.0f);
}

results in compiler error.