Component breaks due to bug, deployment misconfiguration, scaling failure, etc., and your company loses $1M during the outage. If it's a vendor with an SLA, you can (sometimes) get them to cover the $1M. If it's in-house, you're eating it. It's often cheaper to build/run stuff yourself (unless it's so commodified that you're benefiting from amortization across the vendor's many clients), but then you have to be self-insured for problems.