Many newer hearing aids and ALD transmitters have more than one microphone as input.
Noise is one of the most serious problems faced by people with a hearing loss. Normally, hearing aids just amplify sounds, which in noisy situations can actually make things worse for a person wearing a hearing aid.
Multi-microphones are one of the most effective features of some hearing aids because they can provide some level of noise reduction. They do this by using the phase difference in the sound detected by two (or more) microphones to determine which direction a particular sound is coming from.
Such multi-microphone directionality can be used to enhance sounds coming from in front of you and to diminish the amplification of sounds coming from your side or back.
The effect is a very helpful noise reduction that can let you hear and understand speech better in noisy situations.
Some aids (or ALDs) simply allow you to turn on (or off) the noise reduction feature; others allow you to increase or decrease the amount of noise reduction being used; and some even automatically invoke more noise reduction in situations which warrant it.
Some hearing aid users prefer to have control over how much noise reduction is applied, and others might prefer the fully automatic noise reduction.
Even the best noise reduction technology, today, can't match the localization or the directionality capabilities of normal ears, but multi-microphones are one of the most effective noise reduction features available, today.
If you have problems hearing in noise, then at least, ask your hearing loss professional about multi-microphone hearing aids and ALDs.