Using a cleansing oil/balm as your preliminary cleansing step in order to remove all makeup and sunscreen from the skin; therefore allowing your regular cleanser to actually work on the skin and not to have to push through all of the other gunk.
Makeup, sunscreen, and the pollution our skin comes in contact with every day can create an imbalance of free radicals in the skin. Meaning that you may not be producing the protection your skin needs, leading to pigmentation, wrinkles, and a loss of p=collagen production. It's really important to have squeaky clean skin at the end of the day.
People who wear transfer-resistant makeup or someone, like me, who wears an SPF every day that has an ingredient like Zinc in it. Also great for those with oily skin because the oil cleanser will break through some of that extra sebum to help make your second cleanse more effective.