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.