Should I: Backup on RAID, or Backup and RAID? BTRFS specifically.

I researched this a little, but people talk about backup and raid like they should be on separate hard drives. Why? Wouldn't it save drives and be easier just to consolidate the two tactics? My backup strategy is to have local storage (including backup), cloud storage, and cold storage. I'm very new to BTRFS and it seems like it would be nice if I could combine my drives into one big redundant super drive, so to speak. What is the good, the bad, and the ugly of such a tactic? What are your thoughts?

Thanks,
Andrew