why --shebang for rdmd?

[Shriramana Sharma via Digitalmars-d-learn]

Hello. The following code works fine for me:

#! /usr/bin/env rdmd
import std.stdio;
void main() { writeln(2); }

So what is the use of the --shebang option of rdmd? 
http://dlang.org/rdmd.html does not shed much light on this.

Thanks.

-- 
Shriramana Sharma, Penguin #395953