The usage and access of data in your data store when building a realtime application. If you were building a chat-app these days you are given a number of very easy to adopt (minimal complexity/edge cases) NoSQL solutions - Firebase, Parse, RethinkDB.

Whereas with SQL you have the option of using cache layers (which you could say is NoSQL anyway), message queues, naive timely polling and log monitoring.

With NoSQL the solutions are just simpler. For example Firebase has a tutorial for creating a real-time chat [1] - it takes 5 minutes.

[1] https://www.firebase.com/tutorial/#example/chat