I use the stofen to diffuse the flash light. I find when I bounce the flash, the light is more soft and even rather than a hard light.
TBF, you may have a point, but just to be clear, if you use a Stofen when there are no bounce surfaces nearby (say outdoors, in the garden, away from all near surroundings) then because the Stofen is no larger than a bare flash head, there is no softening and the main result is 90% of the light is simply wasted.
However, in some common outdoor flash situations there may well be some impromptu bounce surfaces nearby - such as other people's faces, white shirts etc including your own - and since the Stofen blasts light pretty much everywhere, some of that light can find its way back to the subject and provide a bit of shadows lift. More fill-in than softening, but it can help.
And also, tilting the flash head upwards with a Stofen raises the light source very slightly and that sometimes looks better at close range, too.
This is why press photographers seem to have a Stofen almost permanently attached and in the typical press scrum that we often see on TV news, shooting distances are short so even if most of the light is wasted there's still enough power for a decently fast recycle time at higher ISO. Either way, it's unlikely to do any harm. Stofens are also small, robust and unobtrusive but there are much better and more effective devices IMHO, such as the Lumiquest QuikBounce. It operates on the same basic principle, as all these so-called diffusion devices do, but benefits from making the light source 5-6x bigger, and doesn't waste light out of the back.