Wearing one hearing aid would be the same as wearing eye glasses with only one lens in it. When wearing one aid you do not allow the ears to work together with the brain to sort out the sounds.
As the old saying goes, two heads are better than one. That also goes for ears! But what are the actual advantages? Here's one good reason.
Have you heard about the "Lazy ear"? This is where the unaided ear tends to lose its ability to hear and understand over time. Both ears therefore need to be kept active. That's one reason for sure.