Ceramide originally stays between cells (intercellular) in keratin layer ,which is the outer layer of our skin.
Sphingosine and Fatty acid bounding become the Sphingolipid .40-50% of them are Ceramide。
Like waterproof ,they keep the integrity of stratum corneum of the keratin layer, which prevent water from vaporizes easily. They also help prevent our skin from any bacteria or threaten from polluted area,especially during outdoor activities.
（picture shows：Skin outer layer is composed by sphingolipid in keratin, where 40-50% are ceramide)
Ageing ,drug misuse ,pollution etc will make Ceramide decrease. Hence, you will start to feel dryness and uncomfortable after what have happened mentioned above. After a long while with imblance between the moisture and lipid contents on your outer layer of skin, sensitivity and increasing ageing appearance will be seen.
Hence ,you definitely need Ceramide to recompose the ratio balance on outer layer of your skin.