The number of times that I've watched the bloom encroaching over the top of it and then carefully prodding it to release some gas so it can drop down again. I need to be more patient, I think.
20s gives time to pour hot water and stir. It makes sense when you do it. Start the timer before pouring the first water. Idea is to wet all the grinds - if you fill it right away there may be some dry grinds on the bottom (especially with a finer grind).
Regular coffee you bloom so channels don’t form for all the water to run down.
With the Aeropress, the coffee foams and could foam over if you fill it, so blooming first let’s you stir the bubbles out and then top it off.