A possible implementation of std::merge algorithm

Question

I'm reviewing STL Algorithms. So I've written some algorithms among which this one merge_:

template <typename InIt1, typename InIt2, typename OutIt>
OutIt merge_(InIt1 first1, InIt1 last1, InIt2 first2, InIt2 last2, OutIt dest){

    while(first1 != last1){
        if(first2 == last2)
            return copy(first1, last1, dest);
        *dest++ = *first1 < *first2 ? *first1++ : *first2++;
    }

    return copy(first2, last2, dest);
}

  int main(){

    int a[]{1, 3, 5, 7};
    int b[]{2, 4, 10, 12};
    vector<int> v(10, 0);

    // 1 2 3 4 5 7 10 12
    //merge(begin(a), end(a), begin(b), end(b), v.begin());
    merge_(begin(a), end(a), begin(b), end(b), v.begin());

    for(int i : v)
        cout << i << ", ";
    cout << '\n';

    std::cout << "\ndone!\n";
}

• It works just fine however I saw a possible implementation on cppreference: https://en.cppreference.com/w/cpp/algorithm/merge this way:

  First version
  template<class InputIt1, class InputIt2, class OutputIt>
  OutputIt merge(InputIt1 first1, InputIt1 last1,
                 InputIt2 first2, InputIt2 last2,
                 OutputIt d_first)
  {
      for (; first1 != last1; ++d_first) {
          if (first2 == last2) {
              return std::copy(first1, last1, d_first);
          }
          if (*first2 < *first1) {
              *d_first = *first2;
              ++first2;
          } else {
              *d_first = *first1;
              ++first1;
          }
      }
      return std::copy(first2, last2, d_first);
  }
  

I don't know why such complicated logic?