Clustering also has an advantage when any of the servers needs maintenance. In a single server arrangement, if the server needs to be rebooted in order to add software upgrades or for other reasons, the websites go down for a short while because the host is not available.
In a shared server arrangement, you share the server resources with other websites that are all hosted on a single server. This is less expensive than virtually all other paid options, but it does raise a concern in regards to the performance of your own site operating sluggishly because a neighboring site has a spike in traffic or a poor coding issue and is currently hogging a high percentage of the servers resources.
