I bought an SB700 and I noticed that it came with several accessories (a green plastic, a "transparent" plastic and a brown one). I searched and the transparent one is called a "diffuser".
I happened upon this product:
and was wondering if there was any point in me buying that product if I already have the freebie I got with the SB700.
Thanks in advance!
I don't know if that particular modifier is worth the money, but it is a completely different thing from the diffuder that came with your flash.
The included diffuser is intended to ensure that the flash covers the field of view of very wide-angle lenses (something the flash reflector zoom alone cannot do). It is not intended to soften the light in any way, just to spread it out more. The shadows and contrast will still be just as harsh (except that there may be a little extra fill from room reflections).
Some Nikon flashes (notably the SB800/900/910) come with a milky-white dome diffuser that is intended to provide something like a bare bulb effect, sending light off in a nearly 180-degree hemisphere to take advantage of room reflections.
Softboxes (such as the device you linked to) and bounce cards (and similar accessories like the Lumiquest Bounce) are intended to make the light source bigger in relation to the subject, softening shadows and reducing the contrast between dark and light areas. Umbrellas are used for much the same thing. If you have to use an on-camera flash, then a softbox can vastly improve your flash pictures. Keep in mind, though, that they're just as useful off-camera, and getting the flash off of the camera when you can (or bouncing the flash off of a nearby wall or ceiling) can do much more for your pictures than an on-camera softbox.
A softbox like the one you linked to will work well with your SB700, provided you move the strobe off you camera. Photos taken with a strobe directly on the camera hotshoe give results like an ugly drivers license photo.
There are two problems with an on-camera flash:
1) its too close to the lens' centerline, directly causing red-eye and the direct flash leaves no shaddows. We need shadows to see a 3D image in our heads.
2) its too small. Small light is harsh. The little white diffuser that probably came with your SB800 does not make the light bigger, it bounces a bit off the ceiling and walls, but mostly it just costs you a stop.
For tons on off-camera lighting, go to http://strobist.blogspot.com/
There's a difference between a softbox and a diffuser. I imagine that what came with your flash was a diffuser which is nice for being compact and making your light more omnidirectional than a beam of light. The softbox is good for making your light source larger and the larger your light source the softer the light.
If you don't know what you want to do with it you can either drop the money and play around or, and this would be my recommendation, utilize what you have until you realize what it can't do and you'll learn more about your equipment in the process.
