Reverse url mappings is needed when url is to be generated based on controller and the action. It can be implemented using g:link tag or createLink.
When the controller and action name are variables, could be any and we need to get the right url. Then we can use g:link
<g:link controller="$controllerName" action="$actionName" params="[title:myTitle]"/>
The generated url would be "/book/Grails-in-action"
One more thing, since g:link tag generates anchor tag at the end, we can g:link tag around button, image etc. just like the anchor tag.
Hope the above code helps.