What practical effect will setting delegate="true" have in tomcat?

The documentation says (about the delegate flag):

Set to true if you want the class loader to follow the standard Java2 delegation model, and attempt to load classes from parent class loaders before looking inside the web application. Set to false (the default) to have the class loader look inside the web application first, before asking parent class开发者_开发技巧 loaders to find requested classes or resources.

When should this flag be enabled?

What practical effect will the inclusion of this flag have?