Trust model for smart contracts

Hi

I'm trying to wrap my head around the concept of smart contracts. I see that it's a self-executing contract when certain conditions are satisfied. But how can the satisfaction of such conditions are trusted and properly enforced? For example, a casino is supposed to pay out when the buyer wins. It is the casino that decides whether the buyer wins. Can the casino be trusted to honour the (smart) contract? How is trust/consensus established? If the buyer is expected to trust the casino to honour the contract, then why bother with blockchains at all?