[whatsmyusername]

I have found 2 use cases, one of which I've never actually seen in the wild.

The most common use case is, "I need to store data where the schema is unknown or can change without notice, and have my shit not break." This is what we used Mongo for.

The other use case I could see (and this is pretty much only with Dynamo) is, "I want to build an application that's cross-region native. Most of my data is relatively static, so I accept eventual consistency on changes. I will have a separate data store for transactional data and data that cannot be eventually consistent." I want to build this project, but it will never happen because it's too easy to RDBMS in a single region to start.