Perlite is used in soil mixes (including soilless mediums) to improve aeration and modify the soil substructure, keeping it loose, well-draining, and defying compaction. Different quantities can be added depending on the type of plant you are potting up. For example cacti and succulents would prefer more perlite in their mix, opposed to a philodendron which would prefer a bit less as they prefer a bit more water retention.
 
Perlite can also be used on it's own as a propagation medium and is quite a popular choice overseas with plant enthusiats.