The Groovy language serves the need for a more dynamic, flexible, powerful version of Java. It integrates better with Java than any other dynamic language on the JVM and is very easy for Java developers to learn. In fact, since almost all Java code is also valid Groovy code there is almost no learning curve. So, rather than waiting until Java 10 for features such as closures, you can have them now with Groovy.
Grails, the web framework, fills the need for a way to develop Java based web applications without wasting countless hours fighting the complexities of Java, the Servlet API, and the many XML configured technologies that are usually involved. Frameworks like Spring and Hibernate have gone part of the way towards filling this need, but Grails picks them up and takes them much further. So, if you want to build web applications that run in a Java container (Tomcat, Jetty, GlassFish, JBoss, Weblogic, OAS, etc) then Grails is, in my opinion, the best choice!
Hope that helps,