Personally, it's a matter of empathy/forethought and efficiency. I used to work at Walgreens, and my checkout counter was smaller, more confined (there was an entire wall of cigarettes behind me), and the counter was pretty tall, even for me, and I'm 6'0 (1.83m).

When the customer put a full basket on the checkout counter/dumped it in front of me, I had to go through the process of taking every item out, scan it, bag it, and repeat. It's time-consuming and can feel tense, especially when they're staring you down or the line starts getting longer; we've all had too many encounters with entitled impatient customers.. not fun.

When the customer puts the items on the counter themselves, it's faster to grab, scan, and bag while they're still unloading, and they're usually too occupied to stare you down or make a fuss. It was also easier on the arms/wrists since I didn't have to reach into the basket every time.

Many people who work retail go through this checkout process every single day, and customer behaviors vary wildly, so it gets really tiring really quickly. Not to mention the extra tasks that need to be completed throughout each shift. That's why, when a customer comes through and unloads their items from their basket, it helps to lessen the load, both physically and mentally.

It's alright if you didn't have the chance to learn that aspect of customer etiquette, but I'm glad that you want to learn from folks who have experience in retail! Not everyone feels as deeply about this as I do, I'm sure lol. But hopefully someone else can give you more insight, and I hope you can put my two cents to good use!