Because having a staffed system is important for actual safety and the perception of safety. (New York has loads of staff by the way, including at gatelines and on platforms)The picture clearly shows he is not using the device for customer service or internal work reasons.
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If the job is that monotonous and isn’t considered safety critical as the ‘insider’ posters have said, why not get rid of the roles at quiet stations like this one? Plenty of metros globally have their gate lines unstaffed
Gateline is an important job in that they are mostly useful in crowd management and emergency response. On the Tube, in confined underground stations, there should be staff who are trained to assist in evacuation and to close stations to the public at very short notice.