Any joinpoint that has been aspectized by the aopc compiler or by a load time transformation is set up to be able to have advices/interceptors added or removed from it at runtime. This is JBoss AOP's first definition of Dynamic AOP. Using the prepare action allows you to aspectize any joinpoint in your application so that advices/interceptors can be applied later at runtime. The over head of such a massaging of the bytecode is very minimal as it is just an extra boolean expression. The benefits for search an architecture allow you to do things like deploy and undeploy metrics or statistic gathering on a needed basis. If you are using AOP for testing (See "Testing with AOP"), it allows you to deploy/undeploy testing aspects as you run your automated tests on your live system.
JBoss AOP has the ability to apply interceptors on a per instance basis rather than having interceptors be applied entirely to the class. This is very useful when you have instances of an object that need to behave differently in different circumstances.
A perfect example of this is JBoss Cache AOP (TreeCacheAOP). It uses AOP to prepare classes so that field access may be intercepted. When an object is inserted into the cache, TreeCacheAOP adds field interceptors for that particular instance so that it can do automatic replication across a cluster or to automatically provide transaction properties to the object's state. When the object is removed from cache, the field interceptors are removed from that particular instance.