Synopsis is summary of your story so the purpose of it is keep everything together and take control of shootings.It is a preview of your work.Thats why it is very important and you should be careful while doing it.Also be careful about what it should be in it, what it conveys.That is the thing, that is going to show you how your film is going to be.It should convey the story and what is telling in there, who are the characters, what is changing begining to end and what is happening in the end.While doing all these also there are other things you should care about like writing clearly, pointing your thoughts right and you shouldn´t forget that it is just a summary so you shouldn´t stuck with little details.It will not be good for summary and it will be a waste of time and also another reason doing it, synopsis, is avoid wasting time.You do not need too much explanation.If you are doing this, you will be far from the purpose of synopsis.