According to Randall S. Hansen, a writer for Quintcareers.com, in 2000, people with master's degrees made around 20 percent more than people with just bachelor's degrees, and those with doctorates made around 90 percent more. While these figures are 11 years old as of this writing, it's likely that they've held steady, and they highlight the fact that people with graduate degrees often make more money than those without them. This doesn't mean that having a graduate degree will automatically grant you a pay rise, but on an aggregate level, those with graduate degrees are higher earners.
Some jobs are simply not available to people without graduate degrees. This is particularly true in areas including medicine, law and business. Graduate degrees teach a certain set of skills that employers want to know graduates have, so, if you want to be a doctor or lawyer or enter another field that's directly connected to a professional program, graduate school is not just an advantage but a necessity.
Graduate school is also imperative to have a career as a professor. While there are some professors without doctoral degrees, they are the slim minority. In general, universities exclusively hire PhD graduates for academic positions, because the majority of a professor's job is researching and writing. Successful completion of a doctoral program is evidence that someone is able to spend years doing precisely this. What's more, completing a PhD thesis serves as further, applied evidence of graduates' actual skills in terms of what they can produce.
While it's tempting to look at graduate degrees solely in terms of their actual, tangible benefits, the most important reason to go to graduate school is to gain knowledge. Master's and other high-level degrees are devoted to studying a specific topic at a very high level; it's essentially impossible to graduate from one of these programs without extremely intimate knowledge of this topic. While the knowledge you receive from a graduate degree can often (but not always) be applied toward different jobs, higher-paying jobs, or both, it's important to remember that the real point of the degree is the knowledge itself.