What really grates is the whole aggressive shouting that goes on by staff in the area
You are a regular contributor to a professional aviation forum and (apparently) a frequent flyer. Those shouting instructions are telling you nothing you don't know already and therefore constitute an unwelcome intrusion for you. But many of your fellow passengers fly one round-trip per year or less. They don't know which items have to be removed from luggage for scanning. They may have misjudged their timings and find themselves grateful for the queue-combing system. They may never before have encountered an e-passport reader and have no idea what they need to do.
Those shouting instructions and combing the queues are there to defuse the lengthy delays which these occasional travellers inadvertently cause. And by expediting these passengers through those staff help the rest of us. Queue flows at MAN have improved substantially since their introduction. I wouldn't like to revert to the previous chaotic lines we used to see. Also, whilst the 'shouting' staff have to make themselves heard, I've always found them to be pleasant and good-humoured. We're much better off with them than without.