1. RAID1 for availability, so a drive failure won't knock out your server. At the worst you'll be down for 30 mins while they power off and replace the drive; if you have a hotswap bay then you won't be down at all.
2. Backups for disaster recovery where something drastic such as a fried PSU or RAID card firmware has taken out the data on your server.
Backups are a bit like insurance... you realise you need them right after something bad happens, or if you do have insurance sometimes the claim is rejected (the host "lost" the backup or it was "corrupt"). Either way you'll be paying for it.
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