Reflection filters serve to drastically reduce the reflections and standing waves that make up the "sound" of any room.
When recording music, whether voice or instrument, it is very desirable to record your tracks "dry", with as little of the room sound as possible. Reflection filters do this by limiting the amount of sound that gets past the microphone to bounce off the walls and by then diffusing whatever sound does bounce back towards to the microphone.
Essentially, this is a way of treating the acoustic space right around your microphone instead of treating the entire room you are recording in.
There are other reflection filters on the market, almost all of them copies or knock-offs of Post Audio's original designs. Some are inexpensive and not so good and some others are good but crazy expensive. Post Audio hits the sweet spot with filters that do their job very well, are professional quality and are still reasonably priced. That's probably why Post Audio is the largest maker of reflection filters in the USA.