Chunky Organic #4 Horticulture Perlite
The primary reason to use perlite is to help aerate the environment surrounding plants root zones. Plants use oxygen absorbed through their roots to help them metabolize the sugars they make through photosynthesis, part of the process of creating food for themselves.
Perlite holds air and create pockets of oxygen in the soil or water to keep root function on track so plants can get plenty of food.
Dont confuse perlite with vermiculite - another soil amendment. Vermiculite attracts and retains water. It is used to help soils hold more moisture very different from the function of perlite.