The Actor Model is based on the premise of small independent processes working in isolation and where the state can be updated only via message passing. The actors hold the state within themselves, but the asynchronous message passing means there is no guarantee that a stable view of the state can be provided to the calling components. For transactional systems like banking where account deposit and withdrawal need to be atomic, this is a bad fit with an Actor Model.
So, if your Akka applications need to be implementing a shared state model and providing a consensual, stable view of the state across the calling components, STM provides the answer.
To manage multiple transactions running on separate threads as a single atomic ...