Why use Apache as a front-end server? It might seem more convenient to keep things simple, letting a back-end server like Tomcat serve clients directly – and in some cases it is, depending on the type of back-end server and the specific requirements of the project.
If your main server software is secure and fast enough for your environment and provides all the features you need, you probably don’t need a proxy layer. It usually doesn’t make sense to run Apache in front of lighttpd, for example, especially considering that a layer of proxying increases the performance and maintenance load.