Readers often get lost at the transitions between major sections. You need to hold their hand through the process. In a complex manual, you often have to include orientation information in a summary at the end of one chapter and again at the beginning of the next. Hand holding depends largely on how confident your readers are. The more confident and skillful they are, the less they will need or want.
Overviews are very important in long chapters and with tentative readers.
They are also important when people will be reading your manual in a crisis mode. In tutorial manuals, you often state your purpose by telling your readers what they will learn in this chapter. Include any background information that you feel they may need for this chapter alone. In particular, if they need special information from an earlier chapter, be sure to tell them.