Due to stickiness prices almost never drop, but they can grow slower than inflation, which in the limit has the same effect.

In addition to the other comment, the cashier is almost none of the price.

Assuming £10/hour and a cashier that can scan an average of 20 items per minute (which is probably 20 items in 20 seconds and then waiting 40 more for the customer to dig out their wallet), that's 20*60 items per hour = 1200 items per hour = 120 items/£1.

I assume it's mainly useful for other reasons?

This is why I put emphasis on the word "should".