When I'm watching the kids or doing chores I am often watching some "background noise" TV at the same time. If I'm not at home, the phone is the obvious choice. If I am at home, I'm often moving from room to room and it's a lot easier to just take the phone with me than constantly switch devices. And when I'm laying in bed at night feeling lazy, I just watch on my phone instead of moving out to the couch in the living room (sometimes, and sometimes I say in the living room and enjoy my big TV).

I watch more on my phone than on tv - sitting on a train, in bed at night etc

Sometimes you want to watch video, but you don't want to lug an 80-inch TV into your room.