A possible application example, which is why I bought it to me:
If one day photographed outside, one short exposure times is, depending on weather, usual and is pleased with those. However, if you want to carry out a shooting with a studio flash, you quickly reach the dark side of the bright ambient light - because by the flash sync speed can be such flashes often use only 1 / 250s. To get in good weather but at 1 / 250s and still not get any overexposure, you have to forcibly stop down - f / 11 and even smaller apertures are then not uncommon.
With such small aperture but you lost one of the most important ways of creative photography a - the exemption by blurring the background, so shallow depth of field.
In order to counteract the now, one falls back on neutral density filter, this set the ambient brightness by a certain number EV (roughly diaphragm) down. A ND filter (neutral density filter) with X8 does this by 3 aperture (the power of two is important: 2 high 3 = 8), and thus you would in the above example of the f / 11 at f / 4 landed what is already significantly better offers possibilities of exemption.
If this is not enough, so you can still combine Graufilter (as long as it is not a "slim" filters) and as a NDX8 and a NDX2 come together on X16 (they are multiplied), so you can fade in a further diaphragm - already one is at f / 2.8.
Other applications would be, for example, the long exposures a day - will be mapped nice and soft for example with 30 seconds exposure time a waterfall (the water will look like a carpet), so it typically takes quite strong gray filter (X64 and upwards), but you can also take benefit from a X8 this by combining.
Well, although not gone exactly to this product, but I hope you get an idea to use and meaning of such a filter.
Shortly product: The thread is good, is positioned securely and can be solved well yet. The 58mm-diameter fits well on cheaper lenses, I use it with the EF 85 / 1.8 USM and it works great.
