One reason why TAs leave the pan empty on top of the stove is that on occasion the pans come up straight from being washed and aren't fully dried up. This helps evaporate some of the moisture, which, if in excess, could result in sputtering once the oil is added. I don't know if there is any other reason for it, but I do it out of convenience, i.e have the pan on while I'm still getting the last bits ready. I have watched a BIR owner (not chef) do the same in Birmingham and have it even caught on a video, though I'm not sure if I am allowed to share it.