Primary Function of Smooth Endoplasmic Reticulum

In eukaryotic cells, the endoplasmic reticulum (ER) plays a crucial role in the synthesis and processing of proteins and lipids. There are two distinct forms of the endoplasmic reticulum: rough ER and smooth ER. Rough ER is studded with ribosomes, while smooth ER lacks these ribosomes. Given this context, what is the primary function of the smooth endoplasmic reticulum?