BHT is a stabilizer and antioxidant. It helps to keep the product fresh for a long time, preventing the oxidization of components. It is stable even in high temperatures, that’s why it’s an essential component in some products. It also protects the skin from free radicals. As for safety, the component isn't researched very well. It’s considered to be safe in low concentrations. But there is still not enough scientific data collected.
Propylene Glycol is a useful componet with many positive effects. It’s a very good moisturizer. It helps to deliver ingredients into deeper layers of the skin. It creates a protective layer on the skin preventing water loss. The component is researched very well and doesn’t have notable side effects. It’s used even for making food.