1. Hearing aid users cannot use headsets/headphones like normal hearing people. A large number of Apps are based on this functionality.
2. In workplaces, trains, airplanes, etc you cannot use loudspeakers, as you're not expected to disturb others. That takes you backto item 1: headsets/headphones
3. YouTube uploads enormous amounts of content every minute. The trend is increasing, and a large portion of this content requires that it be streamed to the ear (training videos, conferences, etc).
4. Some security codes are sent using audio (for the visually impaired). If you are both visually and hearing impaired (and huge numbers are), this is unfortunate as others may "listen in" on the security code.