There's a huge jump in cost and operational style, to go from two nines to three, because it means you have to have 24/7 support coverage or an on call rotation

There's a huge jump in cost and operational style, to go from two nines to three, because it means you have to have 24/7 support coverage or an on call rotation (and good alerting, or else it's for naught) for nights. Two nines just means you need someone to check their messages sometimes, during the day, on weekends. One nine, and you can forget about the weekends, too—and that's actually sorta OK for certain applications. Three nines also means you can't afford to intentionally take a system down to work on it, or you'll burn all your "oopsie" downtime. That means a ton more work in infrastructure and deployment processes, than two nines.