My troubles with self checkout usualy has to do with the confirmation weight scale which slows things down tremendously. Sam's self check out featured a lack of scale (Sam's also had a scan-as-you-go app my wife loved).
The two places I go to that have self-checkout seem to have disabled the scales. The only thing that is an understandable hassle is having to check my ID for alcohol.
One of those places knows me well enough to authorize the sale before I can get my ID out, which might be a sign that I have a different problem than the hassle of having my ID checked at the self-checkout.
And my biggest problem is that the self-checkouts I see in stores around me do not accept cash (even though earlier there WERE cash-accepting models - in the chain now merged with another). So even if there are faster lines to the self-checkout, that doesn't help me much.
It allows them to confirm what you bought is the same as what you scanned. It also makes sure you didn't miss-count your multiple items. (Either double-scanning an item or failing to scan an item.)
Having now lived in lower-class areas and gentrified areas, stores that can afford a self-checkout kiosk tend to trust poor people less. (At the same time, they over-estimate their losses from theft or accidental shrinkage).
Here they just do random bag checks. When you press ‘pay’ there is a chance you get a notification to wait for an employee to check your bag. They then scan a random number of items from your bag (up to 10) and see if it matches what you scanned.