If you plan to pursue a career in academia or reach the top of some professions, a master's degree is a prerequisite for studying for a doctorate or advancing in a position. Few institutions will consider enrolling a student in a Ph.D program if he does not have a master's degree. Studying for a master's degree will also allow you to develop better relationships with peers in your field and foster links with tutors and other experts in your area of study.
According data from the U.S. Census Bureau, a worker who graduates with a master's degree will earn an average of $2.5 million over a 40-year working life, whereas a graduate who enters the workplace with just a bachelor's degree will average $2.1 million. Professional degree holders do even better, earning $4.4 million over the same period. Any good post-graduate degree will also give you the edge in an increasingly competitive job market.
If you are truly passionate about your subject, the experience of being able to continue to increase your knowledge and hone your expertise is hugely valuable. In modern capitalist societies, the value we attach to education is all too often purely monetary. The worth of your education should be about not only your earning power when you've finished it, but also what you learn, contributions you make in your field and your ability to pass your knowledge on to future generations.
Studying at post-graduate level will help you develop the advanced skills necessary to pursue your academic career further. Even if you decide not to do so, the research, writing and communication skills you learn are easily transferable to the workplace and other areas of your life. You'll also benefit from forging relationships with your peers and tutors who may be valuable contacts when you enter the workforce.