If you want a resilient system redundancy and automatic failover systems are really important, along with solid error handling.
Think about a distributed data store for example. You may spread all your data across multiple distributed areas, but if each area is managing a shard of data and they aren't replications then you still lose functionality when any one region goes down. If you instead have a copy complete copy of data in each region, and a system to automatically switch regions if the primary goes down, your system is much more resilient to outages (though also more complex and expensive).
