Redis disabled, what happens?

I’m running Gitlab CE Omnibus on RPI3, Rasbian Jessie. I noticed that disabling redis in /etc/gitlab/gitlab.rb prevents my machine from becoming unresponsive. What side-effects would this have? The office analogy says “Redis is a communication board with “cubby holes” that can contain tasks for office workers;” but I don’t understand why Gitlab continues to run even without redis enabled. Thank you