Just looking at the image of one in the new society publication on the Goths.
Why a ring pommel?
Is it easier to balance the weight after casting if you only have to trim weight of that otherwise irrelevant bit of iron ring at the bottom ?
Or is there a better idea why?
Easier to hang on the wall, perhaps?
The balance argument is a good one. Another possibility is that a cord or strap attaching it to the wrist could attached. Or it allowed a bigger showier pommel for the weight (i.e. a fashion statement).
I was tempted to say ease of manufacture - you just bend the tang into a circle rather than making and attaching a separate pommel - but in fact not many examples look as if they were manufactured that way.