Any part you have only one of can fail and take everything with it. Redundancy means running spare copies, such as N+1 servers or a database standby, so one failure is absorbed. Stateless web servers usually run active-active, while a database primary often uses active-passive with a standby that can be promoted. Copies that share a zone, a config or a certificate fail together, and a standby you never tested may not work when you need it.