Question about passing function as arguments to function

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C

#include <iostream.h>
#include <conio.h>
void main()
{
  clrscr();
  int sum(int );
  int mynumber=20;

  mynumber=sum(mynumber);
  cout<<mynumber;
  getch();
}

int sum(int number)
{

  number=number+5;
  return number;

}

my question is that from above i think i have change the modify the value of variable actual mynumber with out using passing pointer as arguments to function

then why we pointers as arguments to function