One use case that comes to mind is backups. I can have a zpool created backed by a S3 vdev and then use zfs send | zfs recv to backup datasets to S3 ( or the billion other S3 like providers)
Saves me the step of creating an instance with EBS volumes and snapshotting those to S3 or whatever
haven't done the math at all on whether that's cost effective, but that's the usecase that comes to mind immediately
I hope you are not having that massive storage storage on public-cloud then you would need MayaNAS to reduce storage costs.
For S3 as frontend use MinIO gateway - serves S3 API from your ZFS filesystem
Not sure what is the use case out of my ignorance, but I guess one can use it to `zfs send` backups to s3 in a very neat manner.