Basically, if you have two ears with hearing loss, and if both ears could benefit from hearing aids, you need two hearing aids. It is important to realize there are no "normal" animals born with only one ear. Simply stated, you have two ears because you need two ears. If you try to amplify sound in only one ear, you cannot expect to do very well. Even the best hearing aid will sound "flat" or "dull" when worn in only one ear.
Assuming you have two ears that hear about the same, you can do a little experiment at home to better understand how important binaural hearing is:
First, gently close just one ear, by simply pressing the little fleshy part in the front of your ear canal (the tragus) into your ear canal -- a little. Do not apply pressure, do not hurt yourself. Just close the ear canal to eliminate sound from entering the ear. The idea is to close that ear for about ten minutes while you watch TV or listen to the radio or speak with your spouse. Then, after a full ten minutes, remove your finger. What an amazing difference!
There are many advantages associated with binaural (two ear) listening and importantly, there are problems associated with wearing only one hearing aid -- if you are indeed a candidate for binaural amplification.
Localization (knowing where the sound came from) is only possible with two ears and it is just about impossible with one ear. Localization is not just a sound quality issue; it may also be a safety issue. Think about how important it is to know where warning and safety sounds (sirens, screams, babies crying, etc) are coming from. Using both ears together also impacts how well you hear in noise because binaural hearing permits you to selectively attend to the desired signal while "squelching" or paying less attention to undesired sounds -- such as background noise.
Binaural hearing allows a quality of "spaciousness" or "high fidelity" to sounds, which cannot occur with monaural (one ear) listening. Understanding speech clearly, particularly in challenging and noisy situations is much easier while using both ears. Additionally, using two hearing aids allows people to speak to you from either side of your head not just your "good" side!
People cannot hear well using only one ear. There are studies in the research literature that show that children with one normal ear and one "deaf" ear are ten times more likely to repeat a grade when compared to children with two normally hearing ears. Additionally, we know that if you have two ears with hearing impairment and you wear only one hearing aid, the unaided ear is likely to lose word recognition ability more quickly than the ear wearing the hearing aid.