Scalr scales your sql database, with master-slave separation, replication between nodes, and soon application transparent partitioning (sharding). So if you're not into the whole DBA / sysadmin fun, it's worth it.

It also lets you restore a website version to a previous state if you get hacked, use traditional (cheaper) hosting solutions which burst on the cloud when needed, and manage app versions, backups, etc. in a more organized way then homegrown solutions.