I have tethering. I'm a remote employee and I work from coffee shops, other cities, etc. so I use my tethering connection for work (software) when I can't get wifi.
Tethering makes sense. It seems that may people use that much data directly on their phones and that is what I have trouble imagining.
Videos and music (especially podcasts, one of the ones I listen to is regularly over 300mb per episode with multiple weekly episodes) take up a lot for some people.
I use about 2-3GB/month. Sometimes I notice that my podcasts haven't synced when I'm at the car, and make them sync for my ride.