The PhD is different from other degrees in many ways. Let me count the distinctive features of PhD study...
Your Opportunity to Add a Prefix to Your Name
It is the only academic degree that allows you to add "Dr." before your name. With that ego-boosting title, you can proclaim to the world that you belong to a small, select group of individuals who deserve a nomenclature of respect. That "Dr." before your name transmits your right to be regarded as someone whose expertise in a field of study is acknowledged by the custodians of the higher education system of a country.
Your License to Teach Higher Academics
The doctorate is traditionally your "license to teach" at the postgraduate levels within the university system. No other academic degree can give you that career opportunity. In the US, the PhD is a terminal academic degree--the highest recognition you can get in a subject of study.
Makes You Go Beyond Coursework and Examinations
Unlike other degrees, the PhD is not about attending classes and taking examinations to prove your proficiency. The coursework is only a small component of your program. PhDs are awarded for going beyond coursework and examinations and delving into unplumbed depths in your area of interest. When you work on your PhD, you're expected to know all that has been done in your field of study and go beyond what is already known. You have to conduct fresh and original research that builds on what is already there and creates new knowledge in the subject.
Lets You Set the Pace and Time of Completion
Most academic programs other than the PhD follow a set calendar of coursework and examinations. Some may have additional requirements of internships or clinical applications or even a thesis that needs you to show research skills, but these, too, need to be completed within a given time frame. Once you complete your coursework, you can pretty much set your own pace and timeline for the several thresholds you must cross before you get to the final dissertation defense.
The Comprehensive Examinations are Matchless!
Your coursework ends and your candidacy begins once you've passed your comprehensive examinations. These are like no other examinations you can conceive of. The broad spectrum of knowledge--both theoretical and practical--that you are supposed to acquire, process, assimilate and be able to use and apply is so vast that it covers the entire body of knowledge available in your area of study. Once you've studied for, taken and passed your comprehensive examinations, you can be confident that nobody can dispute your mastery in your field.
The Concept of Candidacy
Another distinctive feature of the PhD study is the idea of ABD (all but dissertation) or Candidacy. No other academic program recognizes and acknowledges that you're basically an expert in your subject capable of teaching at the highest level but you're not quite done with your research and cannot claim the full title until you complete the process of writing and defending your original work in the form of a dissertation.
Mentoring by Your Advisory Committee
One particularly distinctive feature of the PhD degree is the process of one-on-one mentoring that you benefit from while you go through the steps required to get to candidacy and beyond. The system of constituting an advisory committee with a major professor (who may be called a tutor or a supervisor in other countries) who will track your progress, monitor your work and advise you on the scope and quality of your work is unmatched. It is this personal give and take with your superiors that gives you the opportunity to learn so much more than set coursework. No other academic degree gives you this extended degree to gain from the best minds in your area of study.
The Proposal (and Its Defense)
Thinking of an original research opportunity, getting the thought approved after fine-tuning your thought process, and putting it down in a proposal you can defend to your committee: no other program gives you this opportunity and ability.
The PhD is the only program that requires you to conduct a substantial, unique, original research project that will result in the creation of new knowledge. It is the only program that needs you to write a dissertation that is capable of being published as a public document after you have been successful in defending your work as valid and substantial enough to merit the degree.
One final word: As an international student in the US, I would not have been able to afford my PhD degree if I didn't have my teaching assistantship.
The PhD is the only program that pays you to study! Most PhD students I know are employed by their universities as Research or Teaching Assistants. The department or school pays them to teach or help with research while they pursue their PhD degree. The stipend is just enough to cover all their necessary expenses including a modest lifestyle without frills.
The assistantships also give you valued work experience in teaching or research and these can be used to bolster your CV when you're ready to look for your first real job!