Here are a few reasons:

1. Personal Growth.  Some people are life-long learners.  They have an insatiable desire to add to their knowledge reservoir, challenge themselves academically, and experience what they consider to be of the most rewarding pursuits in life:  developing the mind.  For these individuals, a graduate education offers the opportunity to do all of that, and in a structured way that requires class attendance, completion of course assignments, and discussing/defending opinions and research conclusions.
2. Greater Employment Opportunities.  It is clear that in many career sectors (education, business, public policy, social work, journalism, and more), a master's degree is replacing a bachelor's as the minimum education requirement for employment.  Upon entering my chosen profession – enrollment and student services – back in 1980, one could secure an entry level position as an admissions counselor, academic advisor, student services coordinator, etc., with a strong personality and work ethic, initiative and good people skills – she or he may not have needed to have a bachelor's degree.  By the end of the '90s, applicants for these same entry- level positions were not even considered unless they held a master's degree.  While holding a graduate degree is not a guarantee of ultimate success, it certainly opens many more doors in the employment recruitment and search process.
3. Greater Career Advancement. Related to point #2, it is evident that those with graduate degrees are at an advantage for promotion/advancement within their organization.  Earning a graduate degree is evidence of persistence, determination, intellectual prowess, interpersonal skills, and ability to handle a challenging environment. All of these are qualities that are highly sought after by recruiters and selection committees.  An employee, who has demonstrated success in a longer-term educational program that requires stamina, discipline, resilience, teamwork, planning, organization and the ability to work well with others, is going to be in line for substantial growth opportunities in his/her chosen career.
4. Financial Rewards.  While I do not advocate pursuing a graduate degree solely for financial reasons, anyone considering this endeavor would be less than wise if s/he were not wondering about the "return on investment."  For example, according to the National Association of Colleges and Employers (https://www.naceweb.org/job-market/compensation/the-difference-a-masters-degree-can-have-on-starting-salary/), 2019 graduates earning a bachelor's degree in business administration are projected to earn a starting salary of $57,133, while MBA graduates can expect to earn $84,580—a differential of nearly $27,500. 
5. Sense of accomplishment.  When I walked across the platform to receive my master's and doctoral diplomas, the feeling of accomplishment was overwhelming.  No one can take your degree away from you.  It stands as a testament to your ability to pursue and achiever your goals.
6. Greater recognition and credibility.  There is not doubt that many who have earned graduate degrees have failed miserably. But there are countless numbers who have gone on to accomplish great things, and who are afforded respect and recognition they would not otherwise receive.