Let's talk about air leakage. Normally when we talk about air leakage we're tlaking about energy conservation -- cutting energy bills by controlling how much air comes into the home. On this project, Jake is at 0.3 ACH50 (1/10 of the allowable metric by code).
In the garage, Jake is atrying to limit air leakage for another reason: safety.
Since this is the garage, there could be cans of gas, running cars, etc. You don't want to breathe that stuff indoors, so he uses Zip sheathing on a few walls but he's also adding it to the ceiling to divide this space from the space above him. If the air barrier stops working on the ceiling, it could cause a health issue, much more serious than a ding to your energy bill.
Watch how he handles it.