I believe that the IB syllabus for the subjects you are taking is possibly the single most important document in existence. Unfortunately, there are many who barely look at it (or even never look at it). If you want to do well in your subjects, you will NEED to look at the syllabus, probably on a regular basis. Why?
1. What to learn - Part of the syllabus goes through in some detail the exact expectations the IB has of students in terms of the facts, concepts, methods, etc, that you are expected to know (both for final exams and for assessment). You may think that you can just rely on your class notes and your teacher to know what to study, but the truth is that teachers aren't always perfect. It is possible that they might miss something. Or you may not think it is important at the time and overlook it in your study. Or you might miss a lesson and not realise. If you check the syllabus, you will be able to see if there are any holes in your learning, and fill them!
2. What NOT to learn - One of the greatest traps that IB students fall into during their study is assuming that "if it's in the textbook, it must be important." However, this is not always the case. Many textbooks contain superfluous information that, while interesting, will not appear on your exams and would be a waste of time for you to study. Some teachers are equally guilty of going off on tangents (note: Maths teachers may discuss tangents: this is actually relevant) and tell you about things you don't need to know. Check the syllabus to pick out the things that you can dismiss - it will save you a lot of time as you prepare for finals.
3. How to organise your notes - In most cases (but not always - it differs between subjects) the syllabus is divided up with topics of study and then syllabus points of what you need to know. As you can see from my notes, I fully believe that the best way to organise and make your notes is exactly according to the arrangement of the syllabus. It makes it easier to revise, to look things up and keeps interlinked ideas together.
4. Assessment - The syllabus also discusses the details of your IB assessment. By reading this, you will know exactly what is expected of you at each point in your IB journey, how much it is worth, and how long it is expected to take you. It would be the greatest tragedy if someone put all their energy into studying for Biology paper 1 (worth 20%) and then failed their Paper 2 (worth 36%). The criteria for assessment can also be found in the syllabus.
5. Command Terms - While not relevant to some subjects, other will have a list of command terms used in the IB exam questions. If you want to answer the questions properly, you will need to know what they are and what they mean. You won't get the marks for defining something when you were supposed to explain it!
It may seem like a big, useless document that is only important for the teachers. However, a student who effectively uses the syllabus throughout their studies will do much better. If you don't have it, get it!