Above and beyond performance improvement, if there is one factor that drives the use of application servers, its complexity. Web applications in particular are among the most complex software ever developed. People have done spectacular things with Web servers, including highly complex applications — however, these often required monumental efforts and were one-time projects. Application servers are designed to accomplish the same thing on a regular basis and without heroic effort
Behind most of the complexity is the need to communicate with multiple servers, typically database and directory servers and to orchestrate a variety of processing such as XML/XSL formatting, validation by business rules, and transaction management. The best of the application servers (and their accompanying development systems) provide very sophisticated management and control toolsas well as the documentation and supportthat can greatly help to tame the beast of complexity. One of the things you should look for especially in the ServerWatch reviews is commentary about how easily an application server product makes the job of coordinating (in your head as well) all the necessary pieces.