How frequent we need to perform MongoDB backups on AWS?

I'm started to analyze how MongoDB works on Amazon AWS and I feel like I'm missing something fundamental here. From what I'v read on Amazon Storage docs, it looks like Amazon does some backup'ing of their hardware disks automatically. So, if they are able transparently to restore every disk (which stores MongoDB data) then do I still need to care about backup and recovery?

I'm mostly interested disaster or failure recovery issues however its related to hardware failure and it's unclear is Amazon already handling that automatically (using disk mirroring or predefined backuping schedules), or we still need to perform it manually (lock, backup, and then restore some day)? If not then what happens when some disk fails on AWS? Does the data gets corrupted (website gets corrupted and partly functional), we get an email from AWS at night and then we need immediately restore (after receiving an email) database in the morning? :)