Ph.D. degrees are research-oriented and develop and hone the skills to coordinate and execute new research. The ability to fluently orchestrate independent research projects is not something that a master's degree or undergraduate degree is likely to provide. People who complete a Ph.D. program are often enthusiastic about their field of study and enjoy the efforts of their work.
To be a professor at a college or university, a Ph.D. degree is practically a requirement. If you are able to attain a professor position with a master's degree, it is likely that making progress toward a Ph.D. degree will be desired or required from the institution you are working for. Working as a professor often presents the opportunity to perform research along side the teaching aspect of the job.
The Ph.D. degree lets potential employers know that you are capable of hard work and dedication. It also tells employers that you are intelligent and capable of learning new things. This is especially desirable in the business world, as it is continually developing itself and often goes hand in hand with new technology. Ph.D. degrees are also commonly associated with an increase in salary.
Probably the best reason for attaining a Ph.D. degree is that, upon completion, you will be fully equipped to participate in finding the solutions to some of the world's most challenging problems. The coursework will have brought you up to date within your field and prepared you to begin pushing its outer edges by conducting new and innovative research.