DynamoDB costs you nothing when its not in use. It only charges based on actual reads and writes performed. As opposed to a traditional Relational database that always has to be on and has to be scaled up vertically to the maximum anticipated load permanently and with a read replica if you really want redundancy.

DynamoDB gives you redundancy out of the box (your tables are replicated across the three availability zones in a region), the scale is available to you if you have sudden traffic in on demand mode or you can set a limit if you wish to manage costs; your queries may receive errors about being throttled at some point if you approach those limits.

For OLTP workloads, DynamoDB (and a lot of other NoSQL-style, cluster based databases) cannot be beat for performance, capacity, scalability and costs. Which is exactly what you want on the front line of a workload that can receive large amounts of traffic.

For OLAP workloads with unknown query patterns across a variable set of data that can change over time and large table scans, a relational database is king because the actual volume of traffic is low but the size of queries are a lot larger usually.