The Rough endoplasmic reticulum (RER) is a network of tubes present inside the eukaryotic cell. The RER looks roughs as it contains ribosomes attached to its surface. The ribosomes present on the surface of RER are the sites of protein synthesis. The network of tubes of the RER act as channels for the transport these proteins to Golgi apparatus through vesicles in the cell.
Thus RER serves the main functions of protein synthesis and transport of these proteins to Golgi for further processing.
Regards.