I'll add our use(s) of brush and what we've seen use them.
We make piles or lines of brush where we cut trees and do any "limbing". Not going to carry / transport it !! We've used brush to make ground blinds for archery and late muzzy hunters. We usually ( though not always ) make them in & around pines or hemlocks because members think the darker shady areas in them make for better ambush possibilities. In the areas we logged, we made piles in crescent-shapes with the open side facing SE to give deer places to bed with wind protection and sun exposure. Like J-bird said, we put bigger wood on the bottom and pile smaller as we go upward. We had deer move right in the same night we finished logging / piling !! They made quick use of the shelter and the tops for chow.
I also use brush to break up the bases of ladder tree stands. A little camo never hurts !!
Our brush piles also make good protection for seedlings / saplings to get a good start, and as Catscratch said, birds perch and poop seeds of all sorts and that helps to spread diversity. Hopefully those spots will become clusters / islands of saplings and native brush & briars. Grouse seem to like to perch on some of the piles and drum, and I've seen some strutting in the spring and fall to impress females that must be watching. Turkeys seem to like laying eggs up against brush piles at our place too. Hens caught sitting on eggs have been the proof. Some of our piles made in or very near pines & hemlocks have made good winter shelter for deer and grouse during snowy, sleet and ice times. I've seen grouse tracks go right into a pile in the snow, and also kicked them out from under pile limbs and logs. Rabbits too use our piles, but we don't have many due to our location - miles of woods. We've only had them since we started food plotting.
And we use some lines of brush to steer deer past spots we want them to pass - usually for archery purposes.