Normally if you are a fresh graduate or third year medical student, you can skip the learning phase and proceed directly to mastery phase. While it is imperative for old graduates to go through a formal learning phase.
The exception is if you are aiming for a 99, then you need a formal learning phase. The reason is that to insure you get a 99, it is important that you have no weak points. Normally a fresh grad or third year medical student has retained enough concepts that they could pass this exam with just a short prep. But if you are aiming for a 99, the concepts you have retained from past studies may not be enough. And you may need to do a formal learning phase to make sure you understood all the topics that will come out in the exam, not just most of it.
However, it is possible to have a shorter learning phase than old graduates have to go through, as it is most probable that you still remember a significant number of the concepts tested in the exam. Therefore, if you are aiming for a 99, the extra effort can pay off in a higher score in the end.