why do we use dynamic arrays in c++ code example

Example 1: declare dynamic array c++

int main()
{
  int size;

  std::cin >> size;

  int *array = new int[size];

  delete [] array;

  return 0;
}

Example 2: dynamic array cpp

#include <iostream>
#include <cstddef> // std::size_t
 
int main()
{
    std::cout << "Enter a positive integer: ";
    std::size_t length{};
    std::cin >> length;
 
    int *array{ new int[length]{} }; // use array new.  Note that length does not need to be constant!
 
    std::cout << "I just allocated an array of integers of length " << length << '\n';
 
    array[0] = 5; // set element 0 to value 5
 
    delete[] array; // use array delete to deallocate array
 
    // we don't need to set array to nullptr/0 here because it's going to go out of scope immediately after this anyway
 
    return 0;
}