Class breakpoints are used whenever you wish to pause a debug session whenever a execution enters any part of a class. Rather than having to set method or line breakpoints throughout the class, you can achieve the same effect by setting a single class breakpoint.
When a class breakpoint is set on a class, anytime execution runs into a method, a constructor, or a static initializer that is defined within that class the debug session will be paused if the conditions (if any) are met.