It’s massively quicker to use a handset or app to scan your own shopping as you go round the store, packing it straight into your bag(s). To load a trolley, unload it onto the belt, and then pack it into bags means handling it three times, which makes no sense at all.
I worked on tills in Waitrose for a few years before retiring and you are not doing anyone out of a job by self-scanning. All staff are multi skilled and whilst all have different strengths and preferences, everyone in store is trained on tills and on stock replenishment (shelf filling). If there’s no queue at tills they’d often leave only one of 14 checkouts open and deploy the other cashiers around the shop floor or warehouse until a call went out for more hands at tills. The only person with unnecessary work to do is the customer handling their groceries three times (and dumping them on a belt that’s a lot dirtier than your bags, even if that particular cashier is one who takes pride in cleaning theirs).
I wish all stores had scan-as-you-shop. Around here it’s only Waitrose and Tesco that have handsets, released by scanning the loyalty card, my local Aldi doesn’t even have a self-service checkout - I agree they’re only really suitable for a small basket shop though.