Why did Shouko use hearing aids even though she's 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.