I’m sure there’s a million more specific changes to the chemistry of coffee by blooming, but the basics are:

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.

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).