Pure functions in D

[Walter Bright]

bearophile wrote:
>> They can be hot swapped (meaning replaced at runtime), because they
>> do not rely on any global initialization or termination state.<
> 
> This sounds true and interesting, but what can it be used/useful for?

I've heard mention many times that languages like Erlang allow hot 
swapping meaning the software can be upgraded without bringing the 
system to a halt first.