Pumice is used in soil mixes (including soil-less mediums) to improve aeration and modify the soil structure, keeping it loose, well-draining, and stopping compaction. Different quantities can be added depending on the type of plant you are potting up. For example cacti and succulents would prefer more pumice in their mix, opposed to a philodendron which would prefer a bit less as they prefer a bit more water retention.
 
A benefit to using pumice over perlite, is pumice is heavier, meaning each time you water, it won't float its way up to the top of the soil mix. This keeps your soil structure the same throughout the pot.
 
Pumice can also be used on it's own as a propagation medium and is quite a popular choice overseas with plant enthusiats. Don't forget to still use fertiliser as pumice contains no natural nutrients.
 
Extra coarse grade (chunky)
10mm - 20mm sized peices
Natural volacnic rock
Porous to retain moisture