The Blackout Backdrop is designed specifically to stop background light shining through. Yes, it can be used as a backdrop on its own (it would of course be black), but it has three purposes.
1. Block a background light source. Double-sided pillow backdrops tend to be somewhat translucent. When you have no choice but to place them in front of backlighting (from a window, for example), the 2nd side may bleed through.
The Blackout Backdrop corrects this. It is 100% opaque to ensure that the pillow side you are using as your backdrop is not "tainted" either by background light, or by the design on the other pillow side.
2. Reduce conflict with an event's theme. You can't always set up your backdrop against a wall. Sometimes it has to be middle of the room. Since pillow backdrops usually consist of two sides and two designs, it often happens that the design you are not using clashes with the event's décor. The Blackout Backdrop corrects this, providing a continuous black surface that won't clash with the rest of the event décor.
3. Use as standalone backdrop. You can, of course, use it as a background in its own right. Solid black and light-absorbing, it's ideal if what you need is a neutral surface. Bear in mind as well that as a flat color, it can also be used as a Chromakey Panel.
In the images below you can see how well the Blackout Backdrop works. Picture one shows it with half the blackout extended so you can see the difference. Note the bleed-through on the bottom left. Picture two shows it in full extension.