Actually the main reason they exist is to reduce shoplifting.
If you go back in time to when the original super market designs were made, prices where on items with printed labels and the person at the checkout actually did the adding up by entering the numbers!
ie checkouts originally existed because adding up to total cost was a non-trivial job, and then you had to hand over cash and somebody needed to calculate the change, work out how you split that into the notes and coins and count it out.
Overtime these steps were automated way - first with barcodes for cost totalling and then both electronic payment methods and payment machines that take cash and automatically give change.
Then the job just became swiping the items across the barcode scanner and pressing a couple of buttons to enable the electronic payment and it took a while for people to realise the original reason for the job had gone and customers could do it themselves.
From what I have seen, they actually make shoplifting much easier. It's just that the product loss is less than salary and benefits of the cashiers, and I'll bet that the company has some way to recoup the losses.
I have no idea what the next iteration will be, or if this is considered acceptable in terms of profits.
You think I stole it? Get the real cops. I actually paid for everything, enjoy.
Why'd I stop going? Armed private security with more ego than training. Much better options. Absolutely not advice, but usually the right look/pace does wonders. It helps to be defensible.
In "better" areas where people are by default more honest (or have money to spend), the losses aren't that bad.
But stores in less affluent areas have dropped self-checkout completely because it isn't worth it due to theft losses.