Import files contains implementation

[simendsjo]

Why does the di files contain implementation?
Is it to allow CTFE from calling modules?

l.d
===
module l;
int f() { return 1; }

l.di
====
// D import file generated from 'l.d'
module l;
int f()
{
return 1;
}

The documentation for this says "They can be used to hide the source 
code", but this is a bit misleading.
http://www.d-programming-language.org/dmd-windows.html#interface_files