Just to be pedantic you can use SQL to query NoSQL databases, just look at the PartiQL ExecuteStatement API for DynamoDB. I assume your question is about why anyone would use a NoSQL databases instead of a relational database. The short answer is scale. If you have well define access patterns that rely only on a primary index, you can effectively scale to billions of QPS. If you want to make relational queries, and/or can’t know your access patterns in advance, use a relational database.