> Why replicate data at the VM/disk level when those disks are already provided as a fully redundant system?
That's easy. EBS and similar solutions comes with the price. They're very expensive. Especially, when you need a lot of IOPs. You may be saving on cross-AZ traffic but you will pay ridiculous amount of money on storage. If you have replication you can use attached storage which is way cheaper.
Azure Managed Disks are inherently replicated. But there are other storage solutions (like redundant WebHDFS) you can take advantage of. At the time, the customers I was talking to wanted to have identical deployments in the cloud (to what they had on-premises, down to the number and size of disks).
That's easy. EBS and similar solutions comes with the price. They're very expensive. Especially, when you need a lot of IOPs. You may be saving on cross-AZ traffic but you will pay ridiculous amount of money on storage. If you have replication you can use attached storage which is way cheaper.