Assuming you’re using fresh beans, coffee releases c02 when exposed to water. It’s usually the first step in pourover recipes and you can usually see it pretty dramatically.
The bloom for cold brew is just to prevent the gasses popping on your lid if you try to close it too early or overflowing. If you fill the jar with your coffee, then all the way to the top with your water, this blooming phase will spill over water and most likely the crust of all the coffee that hasn’t saturated and sunk to the bottom yet. No good. The hour I have in my recipe is definitely overkill but it’s just an easy (and lazy) easy unit of measurement to call out.