When and why would you deliberately break parent delegation with a custom or child-first class loader?

Servlet containers like Tomcat use child-first loading so each deployed web app can bundle its own version of a library without the container's version winning, giving per-application isolation and hot redeploy.

Plugin systems and OSGi do the same to support conflicting dependency versions in one JVM. The danger is class loader leaks: if a redeploy leaves a stray reference into the old loader, none of its classes can be unloaded and Metaspace grows until OOM.