In your implementation the commented line is in fact not crucial to sort the array. The thing is, by using swap() your'e doing something more similar to bubble sort - moving curr left until it matches the sorted part. The "actual" insertion sort would instead push all elements right and add curr to the empty space left at pre+1:

// a std::vector<int>& is also a good option - thx comments
void insertionSort(int* arr, int n) {
    for (int i = 1; i < n; i++) {
        int curr = arr[i];
        int pre = i - 1;
        while (pre >= 0 && arr[pre] > curr) {
            arr[pre+1] = arr[pre];
            pre--;
        }
        arr[pre+1] = curr; // now it's important
    }
}

I think this approach is better than swapping because of the reduction of assignments per element sorted - swap() uses a temp variable.