In C++, when should I use the void command?

Code:
#include <iostream>
#include <math.h>
using namespace std;

int increase(int num);
int decrease(int num);

int main()
{
    int n;
    cout << "Enter a number and press ENTER: ";
    cin >> n;
    cout << "Function returned: ";
    if (n <= 0)
    {
        increase(n);
    }
    else
    {
        decrease(n);
    }
    return 0;
}

int increase(int num)
{
    for (int n; n <= 0; n++)
    {
        cout << n << ", ";
    }
}

int decrease(int num)
{
    for (int n; n >= 0; n--)
    {
        cout << n << ", ";
    }
}
I'm learning functions right now it says in my book that using void won't return a value, however I'm a bit confused by this. When and where should I use the word void? If I change the functions to use void it doesn't change anything in the final result. Hope the question makes since.

Sorry if I didn't really explain what I'm doing here.