Clustering is needed for achieving high availability for a server software. The main purpose of clustering is to achieve 100% availability or a zero down time in service.

A typical server software can be running on one computer machine and it can serve as long as there is no hardware failure or some other failure.

By creating a cluster of more than one machine, we can reduce the chances of our service going un-available in case one of the machine fails.

Doing clustering does not always guarantee that service will be 100% available since there can still be a chance that all the machine in a cluster fail at the same time. However it in not very likely in case you have many machines and they are located at different location or supported by their own resources.