I don’t know why people want to tether iPhone too. But we always think in our own way, other people may just think differently.

Some reasons I can think of for all smartphones:

  1. If you share the hotspot of your phone, you need to tell people the ssid and password, but if you share your internet to one exiting router, people may not aware it. I have done this in my office using my android phone.
  2. Smartphone's hotspot is not efficient when connecting more users, because the phone's wifi chip is not for that purpose.
  3. Smartphone's hotspot may shutdown if there is no activity for some minutes.
  4. Smartphone's hotspot drains your battery. But by using cable, it can charge the battery at the same time.