Why is this?

You have to #include <;iomanip> as well as #include
when you use a line of code such as
```
cout << setw(8) << 123456 << endl;
```
but not have to #include when you do the exact equivalent thing in
```
cout.width(8);
cout << 123456 << endl;
```

Use of cout << setw(8) uses a manipulator that is not declared in iostream,
but is declared in include iomanip. The member function cout.width(int) is
already declared in iostream. There is no need to #include
with the use of the member function. Manipulators are more convenient which is why
they are provided.