Certifications in general are used by people to prove that they know the subject. I myself hold MCSE and MCSD certifications.
Primary aim of most of the people who do certification is to get a job or advance in their job. I have found few of the employers who look for certification when they recruit people. They include Microsoft, NIIT and some more.
But most of the companies do value certification alone. A person with little bit of experience and certification combined has higher value in market, when compared with a person with just certification or experience.
Java certification from Sun has multiple levels, which includes basic Java Programmer certification, and advanced Java Professional certification. For JCP, curriculum is knowledge of Java Language Fundamentals. It does not include Servlets or even Java Beans. But you need to know Java and OOPS very well. You should be familiar with packages such as java.lang and java.util.
I suggest that a person should take a brainbench exam, before actually going for Java Certification, as the cost of the certification has gone up considerably.
Take as much practice tests as you can before you attempt the certification exam.