In Koe no Katachi, Shouko Nishimiya uses hearing aids. She's deaf, so can she actually hear something or not? I have read the manga but didn't watch the movie, so I haven't seen any clue about why she is using hearing aids. For what purposes did she use hearing aids in the first place?
I think it's never mentioned exactly, but the only reason she is using hearing aids can be that she can hear some sounds, but her hearing is bad enough to be considered clinically deaf.
We can actually see her several times in the manga reacting to loud shouts and noises.
There are also some pages in chapter 51 that show speech bubbles from her point of view (one of these pages below), so this also might prove that she isn't completely deaf.
Shouko uses hearing aids because she's deaf. They're meant to assist anyone with hearing loss of specific kinds to allow them to better identify sounds.
If you're looking for a clue, in the first chapter of her introduction, she plainly tells the classroom that she's deaf.
Picture taken from Chapter 1 of Koe no Katachi, available for free from Crunchyroll.
I'm a hearing aid salesperson! Shoko has some degree of hearing somewhere. She uses the "universal communication" method which is generally about getting it out and making yourself understood. Some people try only hearing aids, some people try only sign language and some people use both. Visual cues and hearing cues in unison are clearly enough for certain moments of short speech for Shoko where she doesn't have to write or sign.
That being said, when Shouya yanks her hearing aids out and she bleeds on the right side, it's implied that he perforates her eardrum or causes other structural damage. A perforated eardrum is contraindicated in some countries with a hearing instruments which is why she has to wear a left side monaural hearing aid for the remainder of the time.
She may have some degree of hearing, especially if she can hear certain loud sounds as shown in both the anime and the manga. The term "deaf" can be used as an umbrella term for all people with hearing loss, whether that be mild, moderate, severe, or profound. I myself have moderate hearing loss and wear hearing aids, and they help me hear almost as well as a person with normal hearing. Most people with mild or moderate hearing loss prefer to be called "hard of hearing" instead of "deaf" to prevent any confusion for people who aren't completely educated on the topic. Shouko most likely has severe hearing loss due to the fact that she responds to certain loud sounds as I mentioned before.
Even if someone is deaf they may use hearing aids to let them hear a little bit. This will help them not have to be completely deaf. I use hearing aids because I have hearing loss but I was born deaf. I failed my hearing test as an infant. :( so the hearing aids probably assist Shouko. She might not be completely deaf or she would have a cochlear implant. I have hearing loss so I know a lot about this topic.
I have hearing aids myself so I can kinda understand, I guess?
Her condition of hearing must still let her hear some sounds, specifically louder sounds. From personal experience, the hearing appliance can help you hear sounds that you can't normally hear, making it a lot more clearer and reassured. Her hearing is considered quite severe, since she uses sign language to communicate quite often throughout the movie.
Her hearing aids will really help support her, they would make her feel comforted, since they will allow her to hear to some extent and she has something to rely on to help her hear rather than have nothing to help her. She also flinches at loud sounds, because listening to a bang or a general loud noise is like listening to it x10 louder through a clear microphone and then hearing that reflected sound through a pair of earphones and sometimes the noise wouldn't even be that loud, but it has the same effect (not a great experience).
Anyway, yeah, she has severe hearing loss and likes to be supported by hearing aids.