As EBS is a persistent block storage solution where the data on EBS volume is replicated across multiple servers in different AZ’s to prevent the loss of data from the failure of any single component. This replication makes Amazon EBS volumes makes it more reliable.

Normally, the customers who follow the guidance on Amazon EBS and Amazon EC2 product detail pages typically achieve good performance out of the box but there are certain scenarios where you need a higher network throughput with much better IOPS and one of the ways people tend to achieve that is by configuring a software level RAID arrays on volumes. RAID is supported by almost all the operating system and EBS volumes, it is used to boost IOPS, achieve better network throughput, and for higher fault-tolerant storage.

In Short, RAID is used to:

achieve better IOPS

improve redundancy

lower costs