Why are extern(C/C++) definitions and references mangled differently in separately compiled modules?

[Max Samukha]

On Friday, 6 September 2019 at 17:54:51 UTC, Adam D. Ruppe wrote:
> On Friday, 6 September 2019 at 17:42:08 UTC, Max Samukha wrote:
>> That file was silently imported by the compiler (probably, a 
>> bug).
>
> That's by design - the automatic module import lookups actually 
> always look for .di file first, then .d files.

Is there any practical use of having identically named .d and .di 
alongside?