If you're writing a thesis or dissertation, your supervisor is the most important person in your life. And that goes on for several years.
Many graduate programs follow processes that badly need revising. But until universities are willing to provide additional time and resources to work with graduate students, then you have to make sure you are pleasing your supervisor as part of the process.
So why? Why do so many doctoral and Master's students have to follow the wishes of supervisors, rather than being able to work alongside them as equals, in the pursuit of scholarship?
The short answer is that supervisors have the power to move dissertations forward through the various steps so that you can graduate. It's a practical thing.
But I want to point out that the supervisors are not usually at fault if you get stuck in the process. The vast majority of them are committed, hard-working, and eager to see you succeed. They may be overwhelmed by teaching and research and endless committee meetings, but they bear you no ill-will.
So get to know your supervisor. You'll probably find that they will guide you well, offer advice based on their knowledge and experience, and ensure that you reach the goal post. They may be a little slow in responding, sometimes, but it is usually worth the wait.