The rough endoplasmic reticulum is for the synthesis of insulin and other protein hormones in the pancreas whereas the smooth endoplasmic reticulum is for steroid hormone synthesis in gametes.
The RER has a role in protein synthesis because it has abundant ribosomes and is hence present in the pancreas where insulin and glucagon are proteins.
The SER has a role in steroid hormone synthesis which is essential for gametogenesis and hence abundant in gonads.