Why do "they" say to let pancake/crepe/blintzes mixture sit for hour or so before cooking?!

Both of the other are partially right, flour needs time to hydrate, in some sense it is dry, as it sits and the batter rests the flour molecules open up and are lighter and cook more evenly, pancake batter with baking powder and soda needs only 5 minutes rest, ones with no leavening like crepes/blintz's, yorkshire pudding/popovers need more time to rest and doughs with yeast need them to raise with the CO2 gas it produces.

The reason you let crepe and blintz batter rest is different from the reason you let pancake batter rest.

Crepe and blintz batter should rest so that the bubbles created during whisking the batter together can dissipate

The reason you should let pancake and waffle batter rest is so that the gluten can develop which will allow the pancakes to form properly and not run all over your griddle. The longer you let it rest, the more the gluten will develop though so you don't want it to rest too long.

to let the air out of the batter this way when you cook them there wont be any air pockets that form little holes in the Crepe/Blinzers in regard of the pancake they are different those needs to raise a but during cooking ,verses the other 2 those need to stay flat.