Defense in depth.
- admit that no one mechanism will catch everything.
- reduce chance of widespread exploit.
"…In terms of computer network defense, defense in depth measures should not only prevent security breaches but also buy an organization time to detect and respond to an attack and so reduce and mitigate the consequences of a breach."
“Layering security defenses in an application can reduce the chance of a successful attack. Incorporating redundant security mechanisms requires an attacker to circumvent each mechanism to gain access to a digital asset. For example, a software system with authentication checks may prevent an attacker that has subverted a firewall. Defending an application with multiple layers can prevent a single point of failure that compromises the security of the application."