It is quite simple. According to Tiobe, Java is the most popular programming language worldwide today. The fact indicates that companies require more and more programmers who can code in Java. Having a programmer certification provides you an edge over others and hence it is important to be certified.
Thanks for the reply, another question, I studied for the test version 6 and found the content extremely long and with many trick questions that do not necessarily mediate knowledge. Do you think the test is analyzing the best knowledge of the candidates?
Both of us write programs in our regular job. We learnt quite a few things while writing the book, like corner cases in APIs (e.g., behavior of a call to join(0) on a thread) and understanding of unusual aspects and constructs in the language (e.g., how multiple exceptions can get thrown in certain contexts such as try-with-resources and how to deal with that situation) has helped us write better programs. Now I am learning Groovy, and I find that many things I learnt while writing the book helps there as well (especially for debugging mistakes in Groovy code).
In other words, in our experience doing research for writing the book, we learnt quite a lot and we feel we have become better Java programmers. So, we recommend our fellow programmers and friends as well to take-up this exam.