Well, I think that Win XP SP 2.0 is a great example! Application server should host your classes and provide them services. You, as programmer, should acquire application server services by using AOP (abstract oriented programming – Attribute are common use in .NET). Once you declare a service for your class, application server task is to take care of this service. How? Well you really don’t care, its application server responsibility.
Now what's that to do with Win XP SP 2? Microsoft encourages now every one of us to check our applications against SP 2 due to changes that made to several aspects of operation system services. Now imagine that all your code has been hosted in application server. It was application server responsibility to adjust it code to new changes …. Your code that hosted by application server, and just asks for services using AOP, don’t know about low level details of service implementations. therefore your code stays without any change.
Is that abstracting level between your code and other services supplied by operation system, application server provider or MS current buzzword that makes your code more stable over technological changes? If you work in enterprise environment you know for sure that stability is one of the main enterprises concerned. You can't rewrite your applications code every time SP released, new technology announced or operation system changed. But from enterprise point of view, using application servers can reduce significantly code changes but on the other hand adjust new technology ….