Certainty is one of my favorite content concepts. It means your readers already know what to expect from your newsletter.
Think of James Clear's newsletter. It has a 3-2-1 structure (3 short ideas, 2 quotes, and 1 question.) Every time he publishes, you know what to expect structurally, irrespective of the topic.
Same with Alex and Book's newsletter. Whatever book Alex picks up, you already know there will be three excerpts from the book.
What's the point?
Certainty provides reassurance.
Even before you open the email, you already know what you're getting yourself into, how much effort you need to put in as a reader, and how much time it will take.
Personally, if I had only 5 minutes to catch up between tasks, I'd open a newsletter with the structure I am familiar with. Because I know I could read it under five minutes.
Is familiarity such an important factor in winning the newsletter game? I don't think so.
Is familiarity a good enough factor to consider if it suits your structure? Yes.