why should i update a java jdk 1.6 web application and what is the risk to not do it?

Today i received a request to make a plan to update an web application to Java 1.8 and the reason was the web site security.

So i would like to know if is really important to update our web application to java 1.8 and the real problems if we keep our server using java 1.6 ?

best regards