Why?
At The Same Time
We’re going to spend the next few chapters covering some littler things. It will give you an opportunity to catch up on your reviews, if you need it. The information in these chapters will still be awesomely useful, but they’ll also tend to be grammar points you should pick up on and understand fairly quickly and easily. That way you can spend your brain powers focusing on reviewing and catching up if you need it (while still learning something new and exciting).
In this chapter you’re learning an “at the same time” grammar point. You’re learning it simply because it’s a small grammar point, and it will give you more time to study.