Like prescription eye glasses, hearing aids are not going to help restoring your hearing. They help you hear better. Hearing aids are primarily useful in improving the hearing and speech comprehension of people who have hearing loss that results from damage to the small sensory cells in the inner ear, called hair cells.
The most common type of hearing loss is sensorineural hearing loss. Many people who have this kind of hearing loss due to a damage done by disease, aging, or injury from noise or certain medicines.
How does exactly a hearing aid work? It simply magnifies sound vibrations entering the ear. Surviving hair cells detect the larger vibrations and convert them into neural signals that are passed along to the brain.
Just like eye glasses, the more severe a person’s hearing loss is, the greater the hearing aid amplification needed. But be aware that even the greatest amplification can have limits. If your inner ear is too damaged, even large vibrations will not be converted into neural signals. In fact, under this condition, a hearing aid won’t help.
Source: National Institute on Deafness and Other Communication Disorders