It's impossible to prove or falsify of course but lower cost to them from having to pay fewer cashiers in theory shows up for the customers in lower prices. The other benefit is self checkout usually crams more lanes into a smaller area so you can checkout more quickly than waiting for the 2 lanes at best I'd often see staffed outside of peak seasons before self checkout.
So do big box retailers and grocery stores not have any competition then? I don't want to be snarky but it's not exactly a mono or even duopoly space in most towns for grocery stores and big box retailers are more often single choices in medium and smaller towns but above that you've often got a choice of big box retailers to go to too.
Since WalMart and dollar stores compete in the same space to some extent they have competition. And most things you buy at target can be bought from smaller specialty stores that while not direct competition between them all you have same same variety of things available. (and different prices and quality levels so you can make your own trade offs)
I can check out faster by rarely having to queue for the register, and I can scan things faster than a tired bored retail worker and know it's accurate. Self-checkout is also much less likely for me to catch a cold/flu/etc. as I'm neither in close proximity to the employee who was in close proximity to every other customer that week, and is touching every one of my items after touching every other customers' items. And, I can bag the goods the way I want them bagged, either for easier carrying, cold items together, or for how I want it organized to un-bag at home.
So, I consider self-checkout a real plus, not them stealing my labor. The exception is when the system is ill-designed or ill-tuned, so it halts and I'm effectively debugging it for them, except nothing is fixed b/c the employee just logs in and waves it through. With that exception, I still much prefer self-checkout.