MGMAT is Manhattan GMAT, which is a GMAT prep company.
https://www.manhattangmat.com
I happen to be taking the class so I'm just following the syllabus, but a lot of people simply buy the study guides. The problems at the end of each chapter in the study guides refer to the types of problems in the OG that pertain to the topic that we've learned. I think the reason we're not going in through 1 and then the other is so that we have something to study from Post-Class. In the specific review guides the problems are not split up by topic like in the OG.
This is more closely related to how we'll see the Quant problems given to us during the test. There won't be 2 number property problems, then 2 geometry, then 2 such and such.
At least that's my take on it. Plus we're learning different topics each night, so being able to practice a bunch of topics from the OG guide based on what we've learned is helpful. So we're basically tearing down the OG then using the other guides for later. I'll probably buy another book after my prep, so I have new material and new CAT exams.