He said it was because a deaf fan had written about being made fun of for their hearing aid, and he was showing solidarity.
Why do you sometimes wear a hearing aid?
"Some people think it's a kind of sick joke, but the truth is, a fan wrote to me telling me that she was deaf and felt very depressed about her handicap. At that time, we'd released 'What Difference Does It Make', which is all about appearance making no difference to you. I tho...ught it would be a nice gesture to wear the hearing aid on Top Of The Pops to show the fan that deafness shouldn't be some sort of stigma that you try to hide. Basically, I was trying to give her a bit of confidence in herself." 1984, but not sure which interview