They should remind you of what you read. If the annotation is still confusing, read only the surrounding material. If that does not clear up any confusion, then it may be time to reread the chapter. But the annotations should serve as a sufficient review of the material and save you from having to reread whole chapters or sections of your textbooks. Again, these are time-saving techniques that improve your chances for success.
Annotations can also help when it is time to write a paper based on the readings.
The annotated sections should help you find the significant or important sections, and they can also serve as a guide for including quotes and paraphrases in your work. Just be sure to credit the sources in your footnotes and in the bibliography.