Is there a use for a cluster on a single machine?

Having a cluster on a single machine limits the ability to horizontally scale and creates a single point of failure (that of the machine's failure). So for a non-mission critical business, one may choose to have a (hopefully) multi (core or) cpu, single machine with sufficient RAM to host a cluster of instances to save space, cost and possibly energy costs. Ideally two boxes at the minimum with 2 network interfaces each would help bring hardware redundancy for any software service that is being offered in a cluster of instances.

For instructions on how to create a cluster on a single machine, see the documentation here.