A replicated system may be less reliable, but is more available.
That means that in your single computer scenario, if your server goes down, you are toast.
But in a replicated environment although you will have more failures overall, they won't kill the service, since the redundancy will give you a higher degree of certainty that whatever you are running keeps ticking.
These kind of redundancy solutions are normally sold as Highly Available, not highly reliable. The clue is in the name,