Hearing and understanding are to some extent abilities that we have to learn. If hearing loss means that certain sounds for example, in the high-frequency range, are no longer forwarded to the brain, then the brain gradually forgets how to interpret these sounds: it literally forgets how to hear. Part of the hearing spectrum gets lost.

In the case of moderate hearing loss, up to 100% of the lost capability can be relearnt. One of the functions of a hearing aid is to act as a kind of "trainer" for the brain in order to regain lost hearing. For virtually every level of hearing loss, a hearing aid will improve the perception capacity. Modern hearing aids are a piece of high-performance technology. There have been enormous advances in hearing research and acoustic technology. In terms of both form and function, today’s instruments have almost nothing  in common with hearing aids from years or decades ago. What you saw grandma or grandpa wearing when you were a child may have shaped your ideas, but those banana-shaped, whistling aids behind the ear of the grandparents don't meet todays standard at all.

Hearing aids allow the wearer to understand speech more clearly and to hear sounds which would otherwise be inaudible due to hearing loss. Hearing aids expand the restricted bandwidth of the acoustic environment. Soft and high-pitched tones are audible once more, specific sounds and shades of meaning can be distinguished.
In brief, hearing aids provide a better quality of life by restoring access to sensory experiences you thought you had lost forever.
Technology today can give you a hearing experience even while you are cutting the grass. You can listen to a concert or football commentary on the radio, or your favorite music on your MP3 player, or even talk on the phone. Thanks to wireless transmission at the press of a button, you can listen to your personally requested concert, while your ear remains open and receptive to the world around it.