[TomTomXC]

Imagine a 14mm 2.8 on full frame - that's a very wide angle. Of course a 14mm 2.8 on a mobile phone size sensor will not be used a wide angle, as the senor will only cover a tiny amount of the arc behind the lens. While the two lenses theoretically "transport" the same amount of light, the sensor of the smart phone only sees a tiny amount of it in the center of the theoretical image circle. In practical terms of course the smaller-sensor lens will already be constructed to only have a very small image circle and therefore let through nowhere near as much light as the lens constructed for a larger sensor. That's why it makes sense to consider equivalent f-stops for calculations of light amount passing to the sensor, it also has its place in calculation of DOF.