Hearing loss may be worse in certain pitches than in others. When you have a hearing loss, the brain is always plugging in missing information (redundancy), allowing you to understand what is said. This process is very similar to a spell check on a computer. When the brain no longer has enough of a signal to plug in the missing information, the result is poor understanding of speech.
Hearing aids can be programmed to treat your hearing loss in the specific pitches regions that you need. The result is a greater understanding of speech in a variety of environments.