Directional microphones (or d-mics) are so important that every modern hearing aid has them. What do they do that's so important? As the name implies, d-mics are able to amplify sounds in one direction, while not amplifying sounds in other directions. Most current hearing aids achieve this directionality by using two microphones: one pointed toward the front of the listener, and one pointed behind the listener. That's why there are two microphone ports on modern hearing aids - one for each microphone. Press the "Play Animation" button for a visual representation.
Note the wording used above: "d-mics are able to amplify sounds in one direction, while not amplifying sounds in other directions." The backward-facing microphone is simply turned off in directional mode. That means it doesn't make anything quieter or otherwise affect sounds. It's just off. The reason it matters is because in order for a hearing aid with directional microphones to have any effect, the person must stand with the unwanted sounds behind him (the back microphone is off and won't amplify behind the person). Accordingly, the person should also face the sound source of interest (the front microphone is on and will amplify sounds coming from the front). You can get a better idea of what's going on by clicking the "Surround" and "Directional" microphone buttons below to see where the hearing aid's microphones will be sensitive to sounds in space (the blue shaded region.)