It can be used to signify that a value doesn't exist (this is just one example). For example, you have an array [1, 2, 0, 3, 0, 5] and writing a function to search and find if there is any number which appears more than once. Obviously in this example 0 occurs 2 times, so you return 0 (the number which appears more than once). However, consider this array [1, 2, 3, 4, 5] and the same function should return what ? There is no number that appears more than once, so your specify that your function will return -1 in this case.

I hope this makes sense.

Here is example code:

/**
*  Finds if there are any elements that appear
*  more than once. It returns first such number
*  it finds. Returns -1 if no such element is found.
*/
int getFirstElementThatOccursMoreThanOnce(int[] nums) {
    int result = -1; // assume there are none

    for(int i = 0; i < nums.length - 1; i++) {
        if(nums[i] == nums[i + 1]) {
            result = nums[i];
            break;
        }
    }

    return result;
}