RAID is extremely useful if uptime and availability are important to you or your business. Backups will help ensure you from a catastrophic data loss. But, restoring large amounts of data, like when you experience a drive failure, can take many hours to perform. Those backups could be hours or days old, costing you all the data stored or changed since the last backup. RAID allows you to weather the failure of one or more drives without data loss and, in many cases, without any downtime. 

RAID is also useful if you are having disk IO issues, where applications are waiting on the disk to perform tasks. Going with RAID will provide you with additional throughput by allowing you to read and write data from multiple drives instead of a single drive. Additionally, if you go with hardware RAID, the hardware RAID card will include additional memory to be used as a cache, reducing the strain put on the physical hardware and increasing overall performance.