There are a few reasons that I'm aware of to combine soft-box and an additional diffuser:

1. With smaller softboxes and hotter lights (read: flashes) the softbox isn't always able to completely diffuse the light source, which causes a 'hotspot'... Essentially the center of the diffusion 'square' is brighter than the edges. It's better than a 'nekkid' flash, but often you can still see the effects of that hotspot on the subject. There are several solutions (lower flash power, bigger box, move the flash further back in the box, etc.) but often the most expedient way to deal with the problem during a 'live' shoot is simply to throw another layer of diffusion in front of the box to soften out the hotspot.

2. To lower the power of the light without having to rip open the box and fiddle with the flash. If you're within a stop(ish) of where you want the light to be and your light source is a manual flash where the controls are not easy to get at, it's often more expedient to simply lower the light by throwing another layer of diffusion in there than to monkey with the controls on the flash.

3. It is possible to make a small box somewhat bigger (at the expense of power) by hanging a diffuser in front of it... Now you're correct that this is not what's going on with this particular picture (the diffuser is too close to the box to have much effect in this regard), but for the sake of 'completeness' I thought I'd mention it. :-)

For me personally, adding a diffuser is mostly a 'hack' manuver (not hack as in 'unprofessional,' hack as in 'hacker') when I have actual clients in the studio and I don't have the time (or don't want to take the time) to be fussing with the lights while they sit there and wait for me to get it right by 'dialing it in.' If the light is already 'pretty close' but I just need to dial it down 'a bit more,' or 'soften it slightly' it's usually a big time savings just to hack the solution, rather than fiddle with making it 'really really right' by dialing it in at the source, or moving the lights back and forth... again all while the client sits there and waits for me (extra not good if the client is in the short-attention-span set... AKA a child). :-)