With wifi off and my display fairly dim, if I watch a movie I can get 4-5hrs out of my laptop.
With wifi on and my display brighter (like on a sunny day), I'm lucky if I get 3hrs. This is no good on a tcon or longer flight. I know my phone will eat up power if I'm on wifi streaming a movie. Also, some people may still want to use their laptop/device after the flight in a location that doesnt have a power plug.