When you start reading a book, you don't know how valuable it's going to be. Reading slowly and thoroughly might be a waste of time.
Reading in two phases helps with filtering out content that's not useful and engaging more with the content that is. Thus increasing knowledge in the end.
This is also called "the barbell method of reading". The two phases are:
Tags building personal knowledge note-taking
Sasha on Zettelkasten.de (2018): The Barbell Method of Reading https://zettelkasten.de/posts/barbell-method-reading/