Why do we put twice time jar/ejb files in EAR application?

I have a question i'm steel wondering about generation of EAR file with multi-modules projects. Why do we put twice time the jar or ejb files in the ear file? Indeed, usually when we build a .war file the jar and ejb are already included inside since we use/refer to them in the web project, and maven import them since they are considered as dependencies. Maybe, we should define in web project pom file these dependencies as "provided" so that they are not imported automatically ? Thus it will make sens to add them in ear project. More generally it's also a question about interest of multi-modules project. What the interest of seperating ejb if in the end we include them in the project ? Beyond the fact that thus we can change of persistence layer more easily.