Super-obvious answer here, but if you only have one of something you can't upgrade things, reboot, etc., without downtime, and you have a single point of failur

Super-obvious answer here, but if you only have one of something you can't upgrade things, reboot, etc., without downtime, and you have a single point of failure if things go wrong. Having N+1 of each component, ideally with automated failover, makes a huge difference to how easy it is to maintain your system. You'll also enjoy life more when you can wait until the morning to fix something falling over in the middle of the night.