Load Balancing and Clustering is ideal for websites and applications that have outgrown a single Dedicated Server, or for anyone that simply wants to protect themselves against the risk of downtime and chronic website slowness.
Have a high traffic website? There's no easier way to protect against the risk of downtime and slowness than by clustering and segmented web roles. Configuring your web servers, database servers, caching servers, and load balancers in "best-practice" N+1 configurations gives you the peace of mind that no single failure can ever disrupt the flow of traffic to your web visitors.
Clustering can give you serious database redundancy, as you can lose one of your MySQL Proxy servers, or one or two of your database servers, and still be able to service query loads from webservers. Further enhancing availability, is a proper implementation of database logging which means that you can easily roll back erroneous database queries upon request, e.g. “oops, I erased 126 rows in my table!”
There is absolutely nothing good about a ‘long uptime’ on a production webserver, kernel updates, OS updates, and service updates need to be applied as they become available. Since your Load Balancer pair can dynamically adjusts traffic depending on backend webserver availability, keeping up to date no longer necessitates costly downtimes during system or service restarts.
Webservers, although booting from local disks, do not store any persistent data. All website data is mounted from an NFS mount, and is common between webservers. Due to the simple and clean nature of this setup, it’s possible to add or remove webservers by simply installing additional servers, adjusting NFS & Database ACL’s, and bringing the webservers’ configuration files over.
Clustered solutions are built for performance from the ground up. It's not enough anymore to just have fast hardware. Serving thousands of concurrent transactions per second is not easy. To be able to consistently guarantee fast page loads even under massive traffic spikes, server clustering is not just recommended, it's required.