Fungal infections are the cause of most peoples dandruff issues, and the same fungi can result in skin issues in the rest of the face. So probably to kill that off.
It's mostly so the product doesn't mold over. Skincare products are excellent mold cultivation medium: they're stored at ambient temperature (or more), they provide moisture (and are stored in moisture-rich environments) and they're full of interesting nutrients.
Plus most people would ‘double dip’ when using jars so there will be a transfer from the skin into the cream in the pot. If you have a pot which lasts a couple of months that is potentially quite a lot of material transferred from your skin into the dish.
