If you know what is hard light (from flash without diffuser) and soft/diffused light and it's properties meaning hard light = sharp shadows and contrast= dramatic versus diffused light=
soft and pleasing/even effect then I think you will get a halo effect from these two types
of lighting with a hole in the diffuser.
Where have you seen it? In which situation? Was there any other modification or additional gear attached to the flash / diffuser cap?
I can imagine that in order to direct the light further or to create a certain effect of sharp shadows such a hole might be good. Search for snouts, some people here in CS have built their own ones.
I heard this modification long long time before, long before the modern flash has it built in, the little bounce card that can pull out when needed.
The main purpose is to direct some light forward to your subject.
If you want to cut a hole, be sure you also have a original uncut one when you want use it at it is.