As the other poster said, it's meant to be silent reading time.
Some reasons why (loosely paraphrasing internal guidance on how to run meetings and why writing culture matters):
- PowerPoints are lazy/evil, and not good formats for conveying complex ideas; by nature, PPT's encourage shallow thinking, and is thus antithetical to the goal of having rigorous analysis/deep-thinking in making complex tech/business decisions
- Docs should stand on their own, without needing to be presented or walked-through by a person; forcing you to explain your ideas and support them in such a way that any sufficiently motivated reader can follow along and work out what they mean on their own, is again a way to encourage you to be rigorous and thorough (but limiting you to 6-pager format, for brevity)
- Writing long-form paragraphs and full sentences forces you to take time to refine your ideas
- Forcing people to take time to digest the doc first, gives everyone a chance to take notes and formulate ideas/responses to the info/arguments presented; this saves a lot of time as compared to, walking through a doc/slides for the first time, and having a roomful of people encounter those ideas for the first time and to ask questions in real time as they encounter them... reading the doc should hopefully already address the most common questions/critiques, and so you can spend more time on 2nd/3rd-order more advanced decisions or arguments