CachingMostRecentProvider - What is the need for MetaStore DynamoDB Table?

We have a encrypted dynamodb table

Firstly, its important that you are referring to Client Side Encryption, and not server side encryption offered by DynamoDB natively.

For Client Side Encryption, CachingMostRecentProvider does not necessarily mean you must use DynamoDB, you are free to store the keys CMP where you like. DynamoDB as a metastore is just a built in solution for the encryption client which enables you to quickly implement caching.