why are the modalpanels surrounded by a panelgroup?

<ui:composition xmlns="http://www.w3.org/1999/xhtml"
 xmlns:s="http://jboss.com/products/seam/taglib"
 xmlns:ui="http://java.sun.com/jsf/facelets"
 xmlns:f="http://java.sun.com/jsf/core"
 xmlns:h="http://java.sun.com/jsf/html"
 xmlns:rich="http://richfaces.org/rich"
 xmlns:a4j="http://richfaces.org/a4j"
 xmlns:richx="http://richfaces.org/richx">
 <h:panelGroup id="modalPanels" layout="block" style="display: none;">
 <ui:include src="/includes/misc/confirmation.xhtml" />
 <ui:include src="/includes/misc/help.xhtml" />
 <ui:include src="/includes/album/createAlbum.xhtml" />
 <ui:include src="/includes/shelf/createShelf.xhtml" />
 <ui:include src="/includes/index/login.xhtml" />
 </h:panelGroup>
</ui:composition>