How can i build slack clone with redis caching?

I am trying to make a slack clone using node.js, redis, mongoDB and socket.io. I have read that slack is using redis in their app and i am wondering what's the reason behind this? I am struggling to understand the caching flow!

Is this diagram correct...???

• What are the advantages of caching chat messages when users send new messages every second?
• If you cache too many messages on redis wouldn't this be a problem?
• What if server go down or some error occur? Am i going to miss all the data from redis?