Moving data back and forth between disk and RAM has a huge impact on performance. It also adds significant complexity to your code, which costs time and money to develop and maintain. So if your data fits in RAM, putting code and data in the same process will eliminate a lot of the complexity and performance bottlenecks.

Think about redis with lua scripting. Data lives in RAM and the scripts can operate directly on the data. Now, instead of lua, imagine your programming language of choice. Next, imagine any representation of the data, not just the data structures provided by redis, also defined using your favorite language. Now imagine a highly available cluster and a durable, distributed streaming backend, all available as a service running in the cloud. This is memstate.io and it's going to be awesome! - Streamed persistent state as a service.