Why using wrappers for D?

[Chalix via Digitalmars-d-learn]

On Monday, 3 October 2016 at 13:51:28 UTC, Mike Parker wrote:
> // wrapfoo.d
> import foo;  // import the foo module from above
>
> void myFunc(string s)
> {
>     import std.string : toStringz;
>     my_func(s.toStringz());
> }

Thank you for the example, Mike!

And thanks to all others who support me with their answers! I 
didn't expect so much answers, the D community seems to be very 
helpful :)

But there still is one thing, which I don't get:

If I "import foo;" in my project, it will be compiled alongside. 
So there is no need for an extra library. Same should be for 
wrapfoo.d. If I "import wrapfoo;", I should just need the 
C-library "foo", and no D-library "food" right?


To have a more practical example, I looked up the "header" of the 
GtkD gtk/Main.d file. There are functions exactly like you 
described:

>public static void init(ref string[] argv)
>	{
>		int argc = cast(int)argv.length;
>		char** outargv = Str.toStringzArray(argv);
>		
>		gtk_init(&argc, &outargv);
>		
>		argv = Str.toStringArray(outargv, argc);
>	}

This function wraps the C-like gtk_init function to a D init 
function.
The gtk_init function is the function from the GTK+ library, 
which is loaded in the gtkc/gtk.d file:
> Linker.link(gtk_init, "gtk_init", LIBRARY.GTK);
Linker and link are defined in the gtkc/Loader.d

So, why is it not enough just to "import gtk.Main"? What kind of 
code is inside the gtkd-3 library?