In networking, using data means using bandwidth (a fraction of the total network speed). It is similar to renting rooms in an apartment building. When you rent a room you are using space in the building. Since the life time of the building is finite using space in the building costs money. Similarly when you watch a video you are using a fraction of the total network bandwidth. The higher the resolution of the video the higher the fraction of the total network bandwidth is used. The longer you watch the higher the fraction when averaged over a month. And similar to a building network equipment life time is finite so using a fraction of the total bandwidth costs a similar fraction of the total network cost.