I'm quite disappointed by most comments so far talking about RAID and data loss. That is not what RAID is for at all.
RAID is for uptime/availability. When a drive fails, the system will keep running and working. For companies, that would lose thousands of currency per hour with a downtime, this is super important that the system keeps running. At home, it's convenience that you can order a new drive and replace without hours of setting up and copying before you can watch the next episode again.
Backups are against data loss. If a single drive fails, a RAID fails or you get some encryption malware or an employee destroys stuff on purpose, then everything is destroyed. It doesn't matter if it was a single, any RAID, HDD or SSD. You order a new drive, make a new volume and restore the data from your backup.