The white stuff in professional soil mixes is perlite and/or vermiculite. Perlite and vermiculite are used in the horticultural industry because they both provide aeration and drainage, they can retain and hold substantial amounts of water and later release it as needed, they are sterile and free from diseases, they have a fairly neutral pH (especially perlite which is neutral), and they are readily available, non-toxic, safe to use, and relatively inexpensive. As a rule of thumb, perlite tends to last longer, has a more neutral pH, and functions much better in hydroponics, outdoor applications, lawns and gardens (in part because it is stronger). Nevertheless, for decades they both have been used by professionals, dedicated amateurs and gardeners.
Potting soil is fluffy because it is usually made up mostly of peat moss.