You pre-summarize, because you tell someone something, you tell them why you are telling them. So they can judge whether it is worth reading on. And to tell som

You pre-summarize, because you tell someone something, you tell them why you are telling them. So they can judge whether it is worth reading on. And to tell someone why, you have to state in a sentence or two what you are going to tell them. The good thing about technical documents is that you can just skip the parts you don't want to read. But it is there for the people who do want to.