I've seen geniuses who have cleared PMT on their own without any coaching. But, most of the candidates (including me) need guidance and it also a bitter truth that only about 15-20% of the total strength get through the exam. So, though joining some coaching class doesn't guarantee you success, but you increase your chances of getting through.
In coaching classes, not only your basics are strengthened, but you learn the art of solving MCQs in minimum possible time. You get to know what and how much to study. Regular tests help you assess your weak and strong areas and help you predict (to some extent) your rank in the main exam based on your performance.
Yes, as stated by learned writer in the previous answer, you need to know the pattern and syllabus of the exam before you start your preparation. So, though pattern varies slightly from exam to exam, the syllabus is almost the same everywhere, the only difference is in addition to the PCB in AIPMT/NEET, there's GK in AIIMS and English in JIPMER, Manipal and second round (ToELR) of AFMC and Gandhian Studies in MGIMS.
NCERT textbooks are the best to clear your basics and all of the question paper in AIPMT/NEET used to be drawn from these books from 2010-13. But, in the recent years questions outside these books have also been asked, so it is always better to have some guidance.
If you want to prepare on your own, then you will either join Distance Learning Program (DLP) of some coaching institute or need MCQ books to get an idea about the type of questions asked and to master short tricks.
I've shown both the sides of the coin to you, now it's your choice how you want to prepare.
All the best!
Mayank Datar, Undergraduate Medical Student