Why do member functions that overload the + operator typically have 2 arguments when 1 produces the same result? [duplicate]

Question

I've noticed that when people overload the + operator they often use two arguments when I've been able to simply pass in 1 and get the same result. What is the reasoning behind why the version with 2 arguments is preferred, when doing it the way with 1 seems to yield the same result?

Below are two segments from my code. (I am using the cctype library for the c string manipulation.)

One-argument version:

myString operator + (const myString& rhs){
  int length = strlen(str) + strlen(rhs.str);
  char* buff = new char[length +1];

  strcpy(buff,str);
  strcat(buff,rhs.str);
  buff[length] = '\0';

  myString temp(buff);
  delete[]buff;
  return temp;
}

Two-argument version:

friend myString operator + (const myString& lhs,const myString& rhs){
  int length = strlen(lhs.str) + strlen(rhs.str);
  char* buff = new char[length + 1];
  
  strcpy(buff,lhs.str);
  strcat(buff,rhs.str);
  buff[length] = '\0';
  
  myString temp(buff);
  delete[] buff;
  
  return temp;
}