For mission-critical websites...
High Availability (aka HA) is a term used in the web hosting industry to refer to a website's ability to stay online – no matter what. This means that whether it's a power outage, a hardware failure, or just a huge increase in traffic to a popular website – the server stays up and the website stays "available" to the users.
Obviously this is critically important for online businesses. Every moment of downtime costs an ecommerce website money. But even if your site is not doing business, being offline means losing potential visitors. And visitors are everything to a website – otherwise, what's the point of having a website?
Load balanced server clusters use "load balancing" to ensure high availability. In other words, incoming traffic is split between all of the servers in the cluster, according to each's capabilities. And more machines can be added to a cluster to expand it to deal with spikes in traffic.
Additionally, if a server in a cluster goes offline due to hardware failure or other issues, the other servers in the cluster step in to take up the slack, ensure that the website remains available even while the offline server is brought back to service. Similarly, a cluster also helps prevent downtime due to server's being offline for critical maintenance – each machine can be serviced individually while the remain online – and the website remains unaffected.
Of course all of this adds complexity to your webmaster's job – but typically any decent web hosting company will provide control panel software to help make administration of the cluster easy, as well as some sort of monitoring service.