Some outdoors clothing have an inner mesh lining of polyester (or similar) fabric. What purpose does such a lining serve? Does it insulate or is it for transporting humidity from the body?
The liner acts as a climate control mechanism for your body through a reduction of moisture and an increase in circulation. The synthetic fiber content acts as a wick to pull the moisture away from your skin, while the mesh composition acts as a suspension system that increases airflow by maintaining separation between your skin/undershirt and the remainder of the garment. Additionally, the increased surface area of the mesh allows for greater air to moisture contact. The wicking and increased airflow therefore increases the rate of evapotranspiration, which allows your body to maintain a more consistent comfort level without the need to add or remove layers.
The mesh liner is for breath-ability, also makes things more lightweight. That way, the inner layer isn't as dense, and still allows moisture, heat, etc, to breathe through the outer layers, if they are so designed.
It also helps protect waterproof/breathable membrane coatings.