It increases the number of tills available making it quicker to shop. We all know the frustrations of being stuck in a queue with a slow clerk or a complaint/issue clogging up the queue for minutes on end.
They will save the wages, yes, but then they have to invest in more advanced hardware, software and expensive maintenance or licenses, so it's probably not an immediate saving.
Can't really say I think the self-service tills are a negative.