Hello Farid,
There are several things about the PMP Exam Simulator that help to more thoroughly prepare you. First, the length is important! Just like the real exam, the simulated exams are 200 questions and have a time limit of four hours. This is important! Sitting though that many questions at one time is a much different experience than only doing ten or a few dozen questions. This helps to make sure that you have the mental stamina to keep going for the required number of questions. In addition, by doing 200 questions you can see how you do in terms of time. If you're not finishing the practice exams in four hours, that shows that you need to focus on speed (and perhaps additional studies of the PMP material).
I would also like to stress that covering the entire breadth of the material on one practice exam is important. If you break out all of the subject areas individually, sometimes you are able to determine the correct answer by knowing which knowledge area or process area the question is testing you on. You won't have this advantage on the real exam, so you should not rely upon it!
No, you do not necessarily need to complete all of the exams. You should complete enough that you feel like you are prepared. A good rule of thumb is that you should study until you are able to consistently score 80% or higher on the exam simulations. This is not a guarantee that you will pass, but in my personal experience it worked well as a threshold.
I only completed four of the nine simulated exams. By the time I took the first exam, I had already studied the material extensively and scored above 80%. I completed three additional practice exams in order to build confidence that I would be able to consistently perform at that level.
One thing that worked particularly well for me was to focus on questions that I answered incorrectly. For any question that I answered incorrectly (from the exam simulator, my preparation book, or another source), I would study that question to make sure that I understood what the correct answer was, and why that was the right answer. This helped me to focus on improving where I needed it most.