parool.ee

Reliability is the number one reason for clustering. To know that your system is on-line no matter what. If any of your computers fail, the systems remains. It usually costs some quite expensive hardware to go with this to the disks, to have your disks doubled and independent of any single chassis of cable failure.

Performance is some other reason - a cluster has more power as it can quite often process in parallel.

Price could sometimes be the factor surprisingly. When multiple computers are clustered, single node could be less fault-tolerant and cost less.