Brick scattering or random placement of pieces is used in MOCs to create different textures and shapes in environments were the surroundings are random and natural. Any pieces can be used but they are usually quite small so that big shapes or not created which can disrupt the randomness. The pieces that are scattered have no connection to the base of the MOC, which allows them to be positioned in any way.
For example, in the first picture the pieces have been scattered on the ground to create a hillside that has a rocky and uneven texture. This looks very effective because unique and random angles are made which make the rock face look natural. Also, the pieces are all the same colour, which helps them blend in so you don't notice that they are scattered.
In this second picture, you can see the pieces have been scattered in a non-random or ordered way. In my opinion this is not as effective, because it looks like the pieces have just been thrown on and little care was taken.
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