It's super useful for maintenance, for example you can replace and upgrade the drives in place without reinstalling the system.
If it's infrequently accessed data then yes, but for a machine that you use every day it's nice if things keep working after a failure and you only need to plug in a replacement disk. I use the same machine for data storage and for home automation for example.
The third copy is in the cloud, write/append only. More work and bandwidth cost to restore, but it protects against malware or fire. So it's for a different (unlikely) scenario.
If I have 3 disks to devote to backup, I'd rather have 1 local copy and two remote copies, vs 1 local copy with RAID and 1 remote copy without.