It is a series of connected flattened sacs within the cytoplasm of eukaryotic cells. It It is important for the synthesis of proteins.
The main function of the rough ER is to provide a place so ribosomes can produce proteins.
It is the organelle that make the proteins after reading the DNA instructions.
Also, it takes part in the transportation of proteins to the Golgi apparatus.
Without the RER a cell can not synthesis new plasma membrane proteins, lysosomal enzymes, proteins for extracellular secretion and proteins for the Golgi apparatus.