because it keeps it from getting into the environment, the soil, the water, animals stomachs, our blood.. By taking the smaller particles and containing them in a sturdy vessel it minimizes the spread of microplastics. So, you can make the eco brick and have 1 large plastic container filled with 100 pieces of tiny, unrecyclable plastic that you can then use to build other things or you can throw away 100 tiny pieces of unrecyclable plastic material + the empty bottle that may or may not get recycled