The way it was explained to me at my medical was that those questions are often asked just so the airline can have a record of any pre existing hearing damage or problems. That way, if you have any hearing issues later down the track, they can differentiate between problems you've previously had, and possible problems attributed to working for them.
I must say, since I started flying (and it's only been a month and a half) my tinnitus has been pretty atrocious when I'm in a quiet area, especially after first finishing a shift due to all the loud noise!